If you're a strategy fan with no fear of a learning curve, this is a must have.
Keep in mind that even if you enjoy other portable strategy titles, you may want to stay away from Mythologies. Mythologies' depth is more complicated than almost anything else on the system, and it really takes a time commitment to be comfortable with the game. In fact, matches can last hours, which unfortunately ensures online opponents will rarely stick around for a full match (even when an opponent disconnects you don't get the Win).
If you can get past the hardcore learning curve, though, Mythologies is a TBS fan's Heaven. The wealth of modes here is fantastic... Hot seat, DS download play, multi-card play, scenario matches, skirmishes, three campaigns, tutorials, achievements, online play -- the list goes on and on. Mythologies is more feature-packed than most console games; it takes dozens of hours to unlock everything, and even then only the skillful will be able to do so. The game also rewards you every time you play, which is nice.
If you've ever played a game in the genre before, you know what to expect from Mythologies' concept. You start off with nothing but a Town Center and a Hero and slowly build more buildings/units to conquer everyone else on the map. Your units can only build around your Town Center, and unlike in Age of Kings you cannot create new Town Centers (or Castles). Instead, there are one or two neutral town centers on the map that players can take over if they want. Another addition to Mythologies is that you select Gods who, while not visible on the field, can use a powerful ability once. You start off with one God, and each time you Age Up (by researching techniques and acquiring large amounts of resources) you can select one more.
There are three races you can play as (Greek, Norse or Egypt); each race has their own special units and unique strengths. Unlike Age of Kings, there is a very large difference between each race; for example, the Norse race doesn't even build mines or mills to acquire resources (they create Ox Carts, which stand on resources for awhile and slowly acquire them). It would have been nice to have more races, but it's definitely a strong improvement over Age of Kings.
Maps are consistently large, which is mainly why matches take so long. In parts this is good; it requires more strategy and skill to win a match. At the same time, this makes Mythologies not very good as a portable title. If you're hunting a game to play during short lunch breaks or bus rides, don't get Mythologies -- not only does it require a lot of thought process (meaning playing at home is optimal, thanks to the lack of noise), you'll never get close to completing a match. Plus, once you take a break from your DS, it's virtually impossible to come back and try to complete a match without rethinking your strategy for a few minutes.
Combat in general has changed. Ranged units are a lot better, and siege units can attack and move in the same turn. "Myth" units have been introduced, which are large, powerful fantasy creatures. There are now three main types of units; Humans (strong against heroes), heroes (strong against myth) and myth (strong against humans). The rock-paper-scissor idea works like a charm and everything is balanced perfectly. The biggest change from Age of Kings is that every unit has differing amounts of health, as opposed to every unit having 10 hit points. The AI has drastically improved. The game always plays to its races' strengths surprisingly well, and don't be surprised if Mythologies hands you your butt if you crank the AI up to hard.
Gameplay aside, Mythologies' visuals and sound are certainly improved. Everything is a lot more detailed, and Myth units look great and are large on the field. Occasionally this can create a camera issue, but it's no big deal. The visual design is frustratingly generic, however, which keeps Mythologies from being on par with other DS strategy titles. As opposed to each race having a theme song, in Mythologies only one song plays the entire battle (maps have their own songs). The soundtrack is good and voice overs are nice, although hearing the same song dozens of times will understandably get old.
Quibbles aside, this is easily one of the deepest DS games out there, and if you're a strategy fan with no fear of a learning curve this game is for you. It may not be good for portable play, but Mythologies' sheer number and quality of modes is astounding. Mythologies is mostly a winner; just don't expect to be treated to the same fantastic online play or exceptional visual design you can be treated to in other DS games.