I've honestly never been able to find a single writer I've been able to hate with a passion personally. I find Millar hit or miss (more hits generally, but when he misses, it's a catastrophe occasionally). Millar's main flaw is dialog and a tenancy to be a little immature with it. Wanted is my prime example, when I read it I thought it was a joke, but I realized Millar was being serious. It had no meaning or purpose and with characters named 'Shithead' and 'Fuckwit', a living pile of feces (who's line was "death by dysentery") and a Bizarro parody with down syndrome, I just didn't know where he was going or what he was trying to do. Occasionally in his more serious works I find the occasionally immature/weird dialog that takes me out of the moment. But more often than not I find Millar's work to be pretty good. If you haven't read Superman the Red Son you really should as it's a great Superman story by Millar, free of any of the former complaints. I found the beginning of Civil War to be a hit, but the story kind of lost it's punch as it went on. Also his stint on the Ultimates was one of my favorite "re-imaging" of the Avengers. Millar has pluses and minuses to me like any other writer. It's just how many more plus you have than minus that makes a writer likable to me. IMO Millar makes more good books than bad of what I've read.
Kick-Ass has been pretty good so far. It's not necessarily my cup of tea, but it's an interesting story. I never paid mind to any hype from Millar, so I don't really get into that whole inflated ego of his I keep hearing about, but overall it's an interesting book. I'm more looking forward to Marvel 1985 of his. His weakness might be his dialog but his strength definately is his ability to add a bit of "real world" to a fictional medium. He can somehow keep a god like Thor in the Avengers but somehow make it seem like he could be fighting the Wrecking Crew across a Starbucks from you.