Lingering half-way between an expansion and a sequel, if you liked City of Heroes you'll love City of Villains.
However, you may have difficulty finding a noticable difference in the actual gameplay. To me, a long time City of Heroes player, it's tough not to draw parallels between many of the new archetypes and the old ones. At least in 4 out of 5 of the Archetypes.
Stalkers are basically low hitpoint Scrappers who are born stealthy and have moved some of their AOE potency to single target damage (which can be extreme).
Brutes more durable than Stalkers, but have durability closer to a Scrapper. Brutes are not tanks, but the added damage can be a good thing if you prefer quicker battles.
Corruptors are if Blasters and Defenders had a kid. Blaster damage, but (to lesser potency) Defender support powers. Their passive ability (doing extra damage against already heavily damaged targets) does not impress much, but who doesn't like the idea of a Defender that does decent damage for a change?
Dominators are basically Controllers that have swapped out their party support secondary for extra attacks. Cool, but not the best trade. (If it wasn't for their nifty Domination power they'd be sunk.)
And then there's the Masterminds. Masterminds are a distinctly new way you can play the game. You basically do most of your work by summoning one to (eventually) six seperate henchmen to assist you, while you sit back and support at range and with powers. Masterminds are pretty good Tanks because you can keep the enemies occupied with your henchmen, and at the same time they're able to be Support heroes (their secondary is made up of mostly Defender primaries) and their damage is still quite good. Masterminds got it all, the only cost being that you need to micromanage your henchmen who, while being fairly good at staying out of trouble, can get you in trouble if you don't keep an eye on them. (To an extent, this mirrors a comic book Mastermind experience remarkably well.) At the time of this writing, Masterminds outnumber any other archetype by about 2 to 1. This may equalize a bit in time, but I'm betting if you're looking for something distinctly new you may be sorely tempted to join their ranks.
Of course, there's a lot more to City of Villains than just the new archetypes. The various maps (including several PvP maps with actual rewards for beating them), base building, and a wide variety of improvements to the CoH engine (including six issues worth of gameplay improvements, content additions, and tweaks) await you if you were thinking of revisiting the game. However, if City of Heroes just wouldn't "click" with you before, you may find the same problem here.
Some pretty good icing on the cake is that you pay the same subscription fee to have access to both City of Villains and (if you already own it) City of Heroes. It's almost like having access to two games for the price of one, except the two games heavily resemble eachother in the core gameplay mechanic.
At the time of this writing, it's still early in the release, but it seems that City of Villains is a pretty solid product, certainly the best in the series.