I've now played this game twice, in effect; once when City of Heroes was new, and then again when City of Villains came onto the scene.
Both times I enjoyed myself immensely, but as you can see above, there are down points.
However, let's start with the good. Creating your super hero is excellent fun, and with all the permutations available it's not hard to create a genuinely unique-looking outfit. The relative lack of detailed face and body modelling (which works so well in the criminally under-played Everquest 2) is a monir quibble in the face of this. After all, it's the costume that most defines a super hero!
Also, the game is fairly simple to get into and learn to use. It's not completely intuitive, but any MMO game is going to have a necessarily higher level of complexity, and this game does an admirable job of drawing in new players gently.
Looks and sound wise, it hits all the spots. There's an over-riding comicbook feel that perfectly suplements the theme, and it simply feels RIGHT.
However, there are problems. Chief among them is depth, or rather a serious lack thereof. An MMOG is designed to be played for months persistently (hence the month subscription fee, which I personally have no problem with; no single player games has near the level of content and support that an MMOG has), and persistent play requires a constantly evolving learning curve and rewards scheme. City of Heroes/Villains lacks this:
Basically, along with a health increase, going up a level here allows you to pick a new power (every other level), or a new upgrade slot for an existing power (the levels in between). Different character classes simply have different power sets to choose from.
So, you head out into the city and beat up good people (if you're a villain) or bad people (if you're a hero) at the behest of quest givers. They then send you to a new quest giver, who tells you to go and beat some more people up. In between, if you're game, you can beat people up for experience and fun...
And that's ALL you do. It's fun while it lasts, but after a month or so it becomes a tiresome, incredibly monotonous experience.
In summary, if you mant a game to introduce you to the realm of the "Massively Multiplayer Online Role Playing Game", then this is a good start because it's so simple and easy to get into. Unfortunately, you'll soon find yourself wondering why you play it and looking for something with a little more depth and complexity, for basically the same reasons.
So! Have a go and have fun; create some hilarious super heros/villains, beat people up, then move on...