If it weren't for the horrible AI and all the bugs, this game would be the best of the series.
The bad: Horrible, horrible tactical AI. Lots of bugs. Time limit seems short. Time scale (2yrs/turn) takes away from immersion.
The entirely mediocre: The strategic AI
I own every game in the series except for the Mongol expansion to Shogun so I've had a lot of experience with the series. It's hard to say to if that means I'm more lenient of the errors or more critical but I'll try to write as objective a review as I can.
Fans of the series, especially of Rome Total War (RTW), will be instantly familiar with the game and it's objectives because except for a few interesting tweaks, it is very similar in feel to RTW. For those of you who have never played the series, the objectives are quite simple. You start off with a mini-campaign where you have to capture 15 territories. Once you've completed this, it unlocks more playable factions (I didn't count but I think it's around a dozen) and you now have the choice of large campaign where you conquer 45 territories (which feels like it is well over half the map). There are also a couple prerequisites you must meet as well besides the territory (for example, Spain must capture Granada and Jerusalem inevitably pitting them against Egyptians and the Moors)
Where the depth comes in is really in which faction you choose. Each has different units such as longbows, hobilars, and billmen for the English as opposed to Jinetes (a missile cavalry unit), Almughavers (a very good foot javelin unit), and tercio's (a late period spanish phalanx) for the spanish. There are of course units that are common to all factions and some factions are very similar but even where the units are similar, they are in quite a different situation. Take Spain and Portugal for instance which have similar units but Portugal is split into two territories completely separated by Spain and both countries border the Moors, a strong faction.
While the mini-campaign allows you to only choose a Christian faction, once unlocked you can choose from factions that belong to Christian, Islam, and Eastern Orthodox Christian. Christian factions of course have to be careful about warring against other Christian factions because the Pope generally frowns upon this (though not always) and could excommunicate you. This has a number of effects as causing your population to revolt as well as allowing every Christian faction to pounce on you freely. Unfortunately, the AI doesn't seem to be entirely logical in this because I often am attacked by Christian factions and they are usually excommunicated, but it never really seems to make sense. They don't pursue the war aggressively and while if stronger, may take a territory from me, the damage is usually limited and they are now preyed upon by other Christian factions losing more than they gain. The main advantage of being Christian is that you can ask the pope for a Crusade. This has many strategic implications such as allowing you to conquer a far off territory and gaining cheap troops to do it with (apparently zealots are cheap to equip!) or if you choose not to join, you may hope it weakens the target of the Crusade or even a Christian faction you may be at war with as it will at least take away a general and some troops from them. I found this part quite enjoyable as it adds a new dimension to the gameplay though I've read some people find it frustrating. I think if you toy with it and know when to press and when to hold back, it really immerses you in the era. While I won't get into the details, the Islamic factions have a similar mechanic called a Jihad (though you are restricted to territories formerly lost) and don't have restrictions warring with each other as there is no religious mediary like the pope.
Though both of these pieces of gameplay existed in the original MTW, I think they are better implemented in MTW 2.
A big difference from all the TW series is the new choices you make for your cities. You can choose that it be a "city" or a "castle". Cities of course are where you really earn your money and also raise foot and missile militia (generally poor and cheap at first but certainly not always!). Castles is where you train your elite units (generally heavy cav) and generate a little revenue but where they really come into play is their inner wall. No longer do you have to worry about the enemy sieging you and penetrating your outer walls and all is lost. You can now retreat to the inner wall! This lets you regroup and generally hold the fort against a much larger opponent than before. This adds an interesting new dimension to the gameplay and you always (for a price) can convert one type to the other but be careful as you lose all the buildings that may not qualify. For instance a castle can hold a chapel, but if you built a cathedral in a city (which costs a lot of money!), and then convert to a castle, you lose the cathedral. Some buildings, such as city halls don't exist as all. So you don't often want to do this if your city/castle is too advanced as you will lose a lot of prior investment.
Ayway, sadly, it's the siege battles where the inadequate tactical AI really shows. One of the main complaints people have is how the enemy often just stands there under a hail of arrows. This is true of non-sieges too. If you have enough missiles, you can devestate the enemy before it ever takes a step. This is near a gamebreaking bug unfortunately though thankfully the AI isn't always so stupid.
Back to the siege battles for castles, another bug is that the enemy infantry will often abandon half of it's siege equipment and just run up to the walls and attack through one point. This might sound like a smart move but this allows the defender to concentrate his defense in one or two spots thus overwhelm the attacker unit by unit. It wouldn't be nearly so bad but they like abandoning rams too so cav heavy armies will lose half it's ability. Finally, only once have I ever even seen an enemy army bring catapults through the outer wall and attack the inner wall so if you were patient, all you ever need to do is abandon the outer wall and just defend the inner wall. The time would run out and you win the siege.
So those are the main examples of the poor combat AI and it is much much to prevalent to have been "missed" in testing. So it smacks of just another example of the publisher just rushing an unfinished product out the door to get money as quickly as possible.
While less prevalent, there are also pathing problems in both siege battles and regular battles though I think it's due to different reasons. In sieges, it seems that units get stuck on invisible pixels near the walls or sometimes thing there may be a gap in the wall (usually an inside corner) where none exists. So they will just mass up there and die. Also, they made the decision such that there is now no moving through the physical space occupied by the individual soldier, horse, or siege engine. I believe in prior TW series this was abstracted so you generally never got hung up on a friendly unit. This is no more and it is much more difficult to move units through each other and while generally this adds the nice affect of disrupting the formation, it becomes horribly troublesome when it comes to siege engines. The AI tend to like to make large groups of siege engines (I'd venture to say it's another bug) and I counter with cav armies. Unfortunately, in the mad scramble to get out of the way from my cav, the enemy siege engines will get hung up on each other and will literally trap individual soldiers in the middle of siege engines. Unfortunately, whether a bug or by design, it is near impossible to order your unit to hack away at the siege engine thus if you don't have missile troops, you won't ever be able to touch the trapped unit and you will lose the battle on time. Also, it seems to be a lot harder to get your units to charge correctly. I'm told this takes a lot of practice but I haven't gotten the hang of it after a lot of hours trying.
Somehow this review turned into an exposition of the numerous bugs and inevitably they will be fixed but it is a great disappointment and a really poor show of class by Sega to push it out the door before it was really ready. But eventually they will fix it and I think that once it's done it will be a great game.
Before I wrap this up I'll also point out that there are now 5 different agent types you can build, spies, assassings, diplomats, priests, and merchants which some can find tedious managing but personally I enjoy. Again they add to the flavor of the game. A 6th agent is a "princess" which I have seen start a game, but not knowing if it's a bug or bad luck, have never had one appear after the game starts (though the enemy factions still get it). Princesses are especially useful in that you can use her as a diplomat or to lure away an enemy general or even to bolster the loyalty of a friendly general. Well, as long as you can get past the idea that you're usually inbreeding! :)
For those who have made it through this far in my review, I'll point out that yes, there is a "New World" though it's usually so late in the game it doesn't do much for you and I believe you have to be one of two factions to ever go there and the time scale is such (2yrs/turn) that you get people who are literally twice as old as their bio says as I don't think they age at the same rate as the time scale. Thankfully I'm told this is easily moddable but then you are doubling the length of the game. This is good for some people as the game is only about 225 turns long which means you really have to keep moving to capture your 45 territories, or you can mod it and take a more leisurely approach.
So in summary, though the largely what you would expect from an amalgamation of RTW and MTW, there are new and interesting dimensions of the game to make it worth buying. However, due to the numerous bugs on release, you might want to wait and see when (or if) they patch the game before you buy. It's not unplayable as it is but it can be a frustrating experience. If you do get it, hope you enjoy.