A good game, with some rather... Unfortunate technical issues.
Now, I tried to get this game to work, honestly; on three separate installs, several times validating files through both steam and the launcher (which always seem to get at least a 1GB worth of files that fail to validate and must be re-downloaded).
The game has constant "Server connection lost" issues, as well as random crashes. Especially once you get to the 4th continent and are already invested in it - both time-wise as well as just loving the gameplay of it. Possibly also money-wise, if you bought cash.
Most of my time played is from a time before the game was available on Steam. I played a few (very brief) stages recently and found the game hasn't changed too much since then. My numerous uninstalls and reinstalls appear to have reset my time played.
I personally still cannot play the game due to the many technical issues. So take that as a warning.
Now, that said, this is a very entertaining game. The controls are nice and responsive, the character classes are varied and your success rate in combat is largely determined by your timing of skills and positioning, which is exactly what an action RPG needs.
Combat is what you'd expect; you try to chain your attacks, getting as high a combo as possible without getting hit. It's very satisfying to eliminate an entire group of enemies without letting your combo break or time out (which is rather quick).
In case you die, you are given five resurrection scrolls per day. It's worth noting that dying will reduce the durability of your gear, so unless you buy a (rather expansive) repair kit from a merchant (for in-game money), you might have some trouble with that.
The stages are instance-based; you pick up your quests at town, then go to the travel map and select stage, difficulty level, whether or not you allow random players to join in to help you and also whether or not you allow "intrusions".
Difficulty levels are pretty intuitive:
"Normal" is your basic enemies-are-one-hit-kills affair which you'll have to go to at least once for each mission. This serves as a chance to get a feel for the zone, as well as it's boss, more than anything.
"Hard" is slightly harder, but you'll probably still find yourself slaughtering the enemies and not caring if you're hit.
"Expert" is slightly more fun; the enemies have enough hitpoints to survive a couple hits, and deal an almost-significent amount of damage.
"Master" is where the fun is really at. Enemies survive a decent number of hits (but they don't feel like spunges) while dishing out good damage (you'll survive a good couple hits yourself, even as a witchblade with no defensive equipment).
Stage difficulty decrees the quality of your reward at the end of the stage, and also the length of the stage; normal difficulty stages are extremely short, while master difficulty last a good few minutes.
I should mention the hell stages now. They are few, and represent the highest challenge in the game. Obviously intended for a party of players who know what they're doing. Think of them as the "raid" stages of C9, except you have a party of four so your personal impact is much larger.
These stages reward players with extremely powerful weapons and armor.
Intrusions are other players entering your stage as enemies, trying to kill you and then kill the final boss. In order to intrude one must speak with the master of intrusions NPC, pay a fee and then they can enter. You are informed when a player intrudes into your dungeon instance.
Intruders cannot use resurrection scrolls, but must instead re-enter through the master of intrusions NPC and pay the fee a second time.
Killing an intruder gives you a token. A certain number of tokens gets you a crate with some random loot. Killing a defending player while intruding also gives a token, which also grants a crate. Don't know what this one contains, because I didn't bother intruding too much.
For those of you who love PvP, and worry that the occasional intruder will not be enough to satisfy your bloodthirst, there are arenas and tournaments. I don't know how tournaments work, but the arena come in your standard 1v1, 2v2 and tag-team formats. There might be more formats by now, but due to the technical issues mentioned above I cannot check.
Classes are divided into arch-types; there's the hunter, the shaman, the witchblade and the warrior.
The warrior is your standard tank, the shaman can become multiple things (mage, healer or even close-range fighter) depending on their class advance, the witchblade is a fast-moving glass-cannon type that (as you'd expect from a glass-cannon) has little defence and lots of damage.
The hunter is the most powerful. Sorry, but it's true; switching to hunter from Witchblade felt like going from master difficulty to normal.
There is crafting, which is rather simple and not too exciting, but it gets the job done. It can be rather expansive at times, and you might be better off going into dungeons to get your equipment.
That said, alchemy is extremely important because shop-bought potions are terrible, and you will need healing going into Hell difficulty (and quite possibly against some of the high-level Master difficulty bosses).
You can only have one crafting profession on each character, so you'll want alts to pick up the less everyday crafts (weapons, armors) while your mains learn alchemy and cooking.
Speaking of alts, the base inventory capacity in C9 is extremely limited. If I had one complaint (other than the technical issues) it would be this. Inventory extension items are cheap, but if you don't feel like spending the money you can have your alts carry your spare crafting materials for you.
You have 12 character slots, so you don't need to worry about those.
Guild system, mail, auction house, friends list - all included. Probably worth mentioning that guilds can have actual buffs that affect their members, so being in a guild is very beneficial. Guilds also want as many members as they can get because each time a member completes a dungeon he earns his level in guild points - so it's a symbiotic win-win situation.
Guild halls are included as well, which guilds can add items to. Unlike most games, where those serve vanity alone, in C9 guild items also grant a certain buff to guild members.
Now would be a good time to mention the stamina system; yes, your usual "You played too much today, pay us money or you can't play anymore" system. The good news is that the amount of stamina you are given is so high, you can probably play for over four hours before it runs out, and you have weekly "reserve" stamina in case you need it.
I only ever used my reserve stamina on the first day of playing with a character (when stages are very brief). Never actually fully ran out of stamina.
Finally, I think it's time to talk about the cash system. Yes, there is a cash shop. Yes, you can literally buy power in it, among other things. The question is, "Is it a deal-breaker?"
For me, personally, no; the the game asks for only a small amount of cash as a one-time payment to get the (sane) maximum amount of power - about 20$. You can go overboard and enchant your gear, but there is no real reason to unless you want to compete in tournaments or go to Hell mode solo, and I'm not sure whether tournaments allow you to bring your own gear or not.
I have personally never bought anything, and found the game very enjoyable before the technical issues started. My main is a level 51 on a Punisher at the moment, though I have other characters.
Would I recommend C9? Yes and no. If you can get it to work without disconnections and crashes, this is a very fun game you can sink multiple hours into. If you can't... Then you can't, and there's nothing you can do about it.
So, give it a shot, if it works for you then you're a luckier man than I.