Champions Online was one of my favorite MMOs, period. I spent hundreds and hundreds and hundreds of hours in this game, mostly playing with other players like one should in an mmo. I don't actually play there anymore, but felt the game deserved at least an honest review from someone who spent and enjoyed so much time in it. Keep in mind my opinion may be a bit bias because of this, but know I'll be trying to be as fair as possible as a reviewer.
The game has a MASSIVE focus on character customization that puts others in its genre to shame (and I've played plenty of them, trust me). You will spend a good half hour or more messing around with the look of your first character before you actually get into the game. And the best part is you can change it at any time. The appearance of your character is completely cosmetic and unrelated to your character's actual equipment, so you don't have to spend hours and hours and hours trying to find gear that you like the look of. It's already there for you at any time. There are so many options revolving around the appearance of your character and whatever they're wearing, that it's worth trying out alone just to see and believe. The game oozes content, has fun and engaging pvp, cool superpowers, and a lot of lore to help get you immersed in the universe. Combat in general feels like it has impact or effect. The world is very detailed and varied with multiple, large areas (such as the city, the canadian wilderness, undersea, etc.) with plentiful dungeons, bosses, and monster variation to boot. My favorite battles were the ones with epic bosses (yes, the game has those) that didn't require a 30-man group to slay so much as a competent, small party. And I know some people may not understand where I'm coming from, but I'm not a fan of games that force you to grind to level 80-100 to get the end-game. The level cap on this game is (last time I checked) level 40, which is fairly easy to get to within a reasonable amount of time when compared to other mmos. So if you're worried about this being a grindy game, don't worry, the trip is shorter than others.
The game did have problems though. For one thing, the quests are fairly generic. Which is forgiveable, given the date this game was released, but it's not going to astound you with its structure. Pretty standard stuff; kill ten of these guys or go get three of these items, etc. This made the pve element of the game fairly boring unfortunately. The A.I in the game occassionally had something new to throw, but again, do not expect mind-blowing A.I to regroup or use successive abilities in perfect synchronization on you or anything like that (boss fights vary). There's also a -really- painful lack of character development for your hero. It's not like Star Wars the Old Republic where you're making decisions that have an impact on your alignment or anything; everything is set in stone. You can not play a villain, only a hero, which may dissapoint or even turn away some fans of hero mmos. I completely understand as I often wish there was a villain faction to fight the hero faction in the open world. You do get a nemesis at some point, which is essentially an npc villain you make, but that's about it. I personally found it a dissapointing feature, as you're only ever going to see that npc when you get randomly attacked by minions of it and get a quest-item drop to go into a dungeon or see your nemesis during alerts (which are essentially quick-action dungeons or game modes with random party members). The music was great sometimes, but unfortunately had a looping issue which made the game very quiet. The fighting animations, while cool to look at when executed properly, did seem unpolished at times (if you ever played Warhammer Online, you'll have a general idea of what I'm talking about). The voice acting is also inconsistent and either hit or miss.
Finally, the free-to-play model that I have mixed feelings about. After I went gold, I simply couldn't go back to silver out of my need for freeform building (which essentially allows you to structure your abilities as well as color them however you like), and need for more customization options, which does come with it. When it came to pvp, subscribers dominated as freeform allows for some insanely powerful builds that just aren't available to free players. Even though there's a que for pvp for non-freeforms, it still cuts off free players from a massive portion of the community and makes que times much longer. There were times when the free-to-play model worked though. You could buy exclusive outfits, consumeables, travel powers (yes, there's more than just flying and running), hideouts, boosts, and more in the cash shop. None of these were required to really succede at the game, but instead offered either conveinance or added to it. There was some really cool cosmetic stuff in it that was reasonably priced (most of it anyway), and felt like a good exchange to keep the game going.
If I had any suggestions to improve the game in its current state, the main one would be to stop dividing the player population between subscribers and non-subscribers by removing the gold status requirement for freeform (can still keep some power types locked off for non-subscribers). It makes the game feel pay-to-win and just made me irritated getting locked off from my freeform character until I dished out another monthly payment. Also implement a supergroup v.s supergroup (guilds) element so that players can battle it out in the streets whenever their supergroups desire.
I know I complained a lot in this little review, but trust me when I say the game is worth taking a look at. It's far from perfect, but has solid combat, plenty of content, MASSIVE customization, and a pretty decent community (from my experience at least). Game gets a 7/10 from me, despite the fact it may be a bit out of date. Go ahead and try this game out if you're even remotely interested in playing a superhero mmo that actually makes you FEEL super. You got nothing to lose.