Yet another mod that made it on the market
Level Design
The good - There are a fair amount of levels, both built in and custom made, with a great deal of variation between them. Most are big enough to get lost in if you don't know where you're going, so they definitely don't feel cramped. That is until you've backed yourself into a corner and get bum rushed by half the mobs.
The bad - While there is a good amount of actual variation, the levels seem to always work against you, rather than offering any kind of hold out points, or tactical advantage. Monsters take much greater advantage of choke points than you could ever hope to, and barricading yourself into a room is a death wish 9 times out of 10.
The Weapons
The good - there are actually some really great weapons to choose from, with unique designs and purposes, especially since they've added a handful more via DLC. Each class, with the exception of the medic and the firebug (For obvious reasons) has at least 2-3 guns that they can call their own (And by that I mean they can buy them cheaper and do more damage with them than other classes), separated into 2-3 clearly notated "tiers", a la more money for more bang. All guns can be "aimed" with a la either iron sights or guns. A big plus.
The bad - the skins and models are good for a mod game, but still dated looking compared to most big-name releases you could compare them to. Some guns have strange designs such as the flamethrower, which mysteriously has recoil (Wtf?), and the flashlight that can be used from most guns is misleading, as it points to a different direction than the gun actually fires. There is also some strange inconsistencies in the iron sights, where some guns mysteriously take you out of iron sights mode every time you fire, while others doesn't. Small mistakes really, the gun play in this game is pretty good.
Monsters
The good - there are 9 monster classes, including the boss that you see only on the final level of any given game (If you live that long.) It feels, however, like a good number. Each monster is extremely different in how they kill you, ranging from the pawn-like Clot, to the fleshpounder who can one shot you in Insane mode. Nonetheless, anything can be deadly if you get cocky or act stupidly, and most of the time you are forced to be a good, fast decision maker in the heat of the moment if you want to live. Luckily the damage boxes are fair, and those with good head-shot aim, regardless of class, will be rewarded with fast and clean kills.
The bad - horrid spawn points and a few hitbox issues can plague you to no end at times. Seemingly the only requirement for a monster to spawn is that you not be looking at it right that moment. That means that you can whirl around, be completely clear behind you, turn to fight what's in front of you, and be attacked from behind by monsters that magically appeared. On hard and insane modes this can be more than a little frustrating.
Class System
The good - seeing a class system in a First Person Shooter is always a great thing. In my opinion it has now become part of the canon of what makes a FPS a good FPS, with few modern exceptions to that rule. There are now seven classes in the game, each with fairly unique play-styles (Defined mostly by the list of aforementioned guns). They all level up from 0-6, and receive increasingly useful bonuses that are specific to their class. Basically each class receives better and better discounts and damage buffs to their specific gun type, along with some other helpful benefits, such as cheaper armor for the medic, fire resistance for the firebug, and overall damage resistance for the melee focused beserker class.
The bad - Unfortunately this game's strongest point often becomes its weakest point, as the class system, particularly leveling them, is so unbalanced it's almost comical. All classes, with the exception of the Medic and the Sniper, level up via doing damage with their specific weapon type. The Medic levels up by doing X amount of healing to himself or others, and the Sniper levels up by getting headshots. None of this is particularly unfair, however the additional requirements placed on a couple of the classes are so stupidly asinine, that most choose to avoid them altogether.
It is generally accepted that the sniper is easiest to level up, because, assuming you make all headshots, you can easily get 200-300 headshots towards your current quota in a single game, even a single player game. This would take you from 0-3 in about 5 games. After that you're looking at somewhere between 15-20 games for each of the last three levels. Not easy, but not arduous either. The Firebug class can be powerleveled via single player "Bloat-ed" games (A mutator, not considered a cheat, and doesn't disable level gaining), which are slow moving, and can soak up a lot of damage, which really helps accrue all the needed fire damage. The demolitions and beserker classes are also both fairly straight forward, and only require explosives and melee damage, respectively. However the beserker is far easier then the demo, as the amount of damage they inflict on even a single one of the bigger enemies is ridiculous, and the demo has to wait at least a level before they have access to an explosive weapon.
And then we have the Support Specialist, and the Commando. Leveling these two is merely annoying, and ridiculously torturous, in that order. The support specialist, for no discernible reason, has faster welding/unwelding as part of his package of class buffs. Unfortunately, this also means that he has a quota of welding points. It isn't a ungodly amount, especially considering the fact that the increase in welding speed makes it go very fast but still, quite pointless. This on top of the fact that the support class has the exact same burden of damage points needed to level up leaves you wondering why they suffer the extra burden. But then you look at the commando, and you no longer feel sorry for the support specialist. The commando, like the support specialist, has a couple "extras" added to their list of benefits, among them include the ability to see the health of an enemy within a certain range of them, and also the ability to detect stalkers (Invisible enemies), at a certain distance. But like the support specialist, they are punished for the latter of these two abilities. In addition to their damage numbers burden, they are given the task of eliminating X number of stalkers prior to each level up. This may not sound bad from the outside, but you should realize that this really means:
-To throw a real example out there: the commando must eliminate (With commando weapons) 1200 stalkers to get to level 4. It only gets higher from there.
-Unlike the firebug who can turn on all bloats and have a bunch of meat shields to fry for easy fire damage points, there is no non-cheating mutator that turns on all stalkers, or even increases their numbers. That leaves commandos with two options: 1. Play multi-player games to increase the number of stalkers they'll see in a given game, in which case they are only guaranteed some of the stalkers, if any, as other players will probably not be willing to simply defer all stalker kills to them, considering some of them may be commandos and need the kills themselves. or 2. Play single player, in which case all kills are guaranteed, but you are guaranteed far fewer chances.
-Either way, you will probably only see 30-45 stalkers in a given 10 round game, assuming you survive all that way, and all kills are yours. Assuming those things, you would need to play somewhere in the neighborhood of 400 games to reach level 4. The quota for level 5 more than doubles.
Get the picture?
So yeah, that's more than a little annoying, considering how easy some of the other classes are to level by comparison.
Graphics
The graphics leave something to be desired. The player models are decent in comparison with the monster models which are quite dated. The worst part are the textures and the static props that you see lying around the place, which look very much Half Life 1. Ouch.
Physics
Nadda. Forget about it. No destructible environments, no debris to blow around, nothing. You'll fly a bit if a fleshpounder hits you, but that's about it.
Overall it's a great game, especially considering it started as a mod. More than any of the gameplay issues, the game could really use an overall in the connectivity department, as finding a good game to join is usually a crap-shoot, and the favorite server feature doesn't work at all.
Have fun.