My first experience with Harlan Ellison was "The Deathbird." I found it pretentious and cloying. Since then I've read "I Have No Mouth and I Must Scream," which unlike most people who read it I found forgettable (though in fairness, it may be a victim of "Seinfeld is Unfunny" syndrome), and "'Repent Harlequin!' Said the Ticktockman," which I quite enjoyed. I don't think I enjoyed Dangerous Visions and Again, Dangerous Visions as much as many people who've read them, but then I've always had a mixed relationship with New Wave SF.
I've never sought his work out to read further (so I've only read stuff of his that I find in anthologies) in part because I don't find him exceptional, but also in part because by pretty much all accounts he's a thoroughly unpleasant person. He's incredibly lawsuit-happy, and his handling of The Last Dangerous Visions has been appalling. I'm a severe procrastinator myself, so if procrastination were the only issue, I wouldn't care. But it goes far beyond that. He tried to sue NESFA for publishing "Himself in Anachron" (only backing off when it turned out the rights had lapsed, something he should have looked into before suing and something he had no business suing about anyways). Christopher Priest's "The Last Deadloss Visions" and Charles Platt's "The Ellison Appreciation Society," reveal a nasty side of the man which goes far beyond the anthology itself. And I'm sure that he can be nice to people who haven't crossed him, as mentioned above, but to my mind the measure of a person isn't how they treat strangers, but how they treat their friends and enemies, and from all I've read he falls short on the former and severely so on the latter.
So I think Ellison is a decent but overrated writer who got lucky with some good anthologies and screen-writing deals which brought him an undeserved reputation and an overly-inflated ego. And the annoying thing is that it seems to extend to how people talk about him. One of my pet peeves about SF is that a lot of people I know tend towards New Wave stuff (I tend to prefer Golden Age stuff myself), and if I don't mention Harlan Ellison among my favorite SF writers, the assumption is that I must not have read him. People never seem to have "Are you familiar with Harlan Ellison?"; it's always "You should read Harlan Ellison." I have; I just don't find him particularly exceptional, certainly not the the degree that confidence implies.