There are a few who complain that you level too fast in CO. The current cap is 40, and it is not all that hard to hit (unless you make four alts and play them just as much).

It's not grindy at all. Basically, you gain squat from grinding mobs. But the quests give out tons of experience. You can easily get from level 5 (end of tutorial) to level 19 in two days, like I just did with a telepathy build. After that, the game slows down considerably, and each level takes some time, as the experience needed outpaces the missions.

Missions are generally the same sort of fare, collect items from boxes, take out a couple mobs, but the variety in some of the instanced missions is a lot of fun. In the low 20s, you start getting missions where you use your crafting aptitudes to flip switches and such that can make the mission that much easier. Other things like holographic disguises show up infrequently. You can solo 80% of the missions with any build.

There's also a lot of the humor in the story arc texts. Mad cowboy robot amusement parks, ultra-right wing Canadian radicals, kung fu love stories, etc.

PvP is fairly unbalanced. You can do great, or min/maxers will exploit you to death in seconds. But it isn't essential like Warhammer, the game in primarily PvE.

There are no classes in CO, you can pretty much make whatever themed character you want by just grabbing the power from another set. Demons, sword-wielding gunslinger, high-tech power armor, powerful psychic with pyrokinesis, etc. Some sets mesh better than others do, based on your stats, but it is more fun to diversify and make a themed character. As such, the character creator is only beaten by City of Heroes, which benefited from years of additions.

If there is a downside, it is that it is both really easy, and really hard to play with your friends. There are no shards, just instanced maps (100 max). You can switch whenever you like, but you'll probably never see the same person twice. Communication is global, type in a person's handle, and you'll find them on whatever character they are using. That doesn't make it any easier to get used to a usual crowd.

Crafting is actually useful. You can make consumable devices like a flamethrower sort of thing, health items, and equipment that can be better than what you get from normal missions. It's also really easy to get caught up on where you are supposed to be in terms of skill by breaking down items.

All in all, it is a lot of fun. I can't speak for endgame though, because I spent too much time across too many characters (pretty typical for me). They are still messing around with a few things since it has only been a month, and PvP will probably be broken forever, but I'm not much of a PvPer in most MMOs (that's what Warhammer was for).