I avoided playing while some of my group got into it from launch. I only tried it back in February and was, as I feared, instantly addicted. I have both CoH and CoV now, and each has it's strengths and weaknesses, IMHO. CoH hasn't seen as much love since CoV went live, but it already had a year's worth of loving to go on, so it's hardly starved.
I missed a lot of the ups-and-downs of previous patches (and thus avoided the rage-inducing Issue 5 update, which drove some to unhappiness). Many improvements have been made since launch, as I find out whenever I ask my friends about how things USED to work.
I like CoV for it's better engine; it isn't sloppy coding that makes it require more hardware...it simply is a much more complicated set of zones. CoH's starting zones are a nice clean expanse of city blocks...CoV's starting zone is a virtual warzone. It is much more object-dense, with lots of improvised fences, piled crates and uneven terrain. The zones in CoV are generally just more diverse, even if somewhat less expansive.
CoH, however, is just generally more appealing to me in some ways, as I just enjoy being the hero more....even though I think the CoV archetypes (character classes, essentially) are slightly better designed.
Both games suitably support solo, duo and group play with equal finesse. One thing that Marius didn't mention was Badges, I think. Badges are both titles and sometimes they are actually benefits. You get them for completing certain missions, visiting certain locations or accomplishing some goal. For example, you earn one badge for climbing on top of City Hall and standing by the flag (I think that's the Patriot Badge). Another badge comes from defeating 100 members of the Skulls gang. Another badge is awarded for rescuing a mystic from the clutches of the Circle of Thorns. And one badge requires you to earn several other badges to earn it; when you do, you get a permanent reduction in your Endurance use for that character. And so on.
What I truly love about CoH/CoV is the unpredictability about it. There is a grind at points during some later levels, but nothing like I've experienced elsewhere. One of my favorite moments was after my wife and I had been playing for a few weeks, when I was running down a street in King's Row...and came face to face with a giant robot marching down the street, followed by an army of clockwork robots. It was a Paladin, and bore the classification of Giant Monster. And oh, how it kicked our butts. But it was super-fun. CoH/CoV has lots of moments like that.
A special set of missions is the Task Force (or in CoV, I believe it's a Strik Force): basically a set of linked missions with a central story that often gives a nice reward at the end...and usually features a battle with an arch-villian like the Clockwork King. They're extremely fun, as they require a minimum amount of characters to even attempt, provide lots of experience (in general, I've usually gotten two levels, per TF).
Death carries a minor sting; when you die, starting at 10th level, you get Debt. Debt is basically an XP penalty for coming back to life that you pay as you go. Until you're clear of it, half of you xp goes towards your debt. It's a very light penalty and I like the system.
I'm normally on Justice. My main characters are:
Bizarregoyle: Level 27 Stone/Stone Tanker
Professor Mesmerize: Level 22 Illusion/Storm Controller
Captain Intrepid: Level 12 Energy/Energy Defender
Vox Avenger: Level 8 Sonic/Energy Blaster
Nova Sword: Level 12 Broadsword/Invulnerabilty Scrapper