Well, many people say the kelly is neck heavy and a lot of people disagree. I think it just sits differently than most guitars.
Some people claim if they let the neck go, it'll nose dive, but that might be due to the fact that they prefer a guitar that the neck tends to point up, like a KV tends to point up at a 45 degree angle, and a warrior a bit less than that, and a strat about level with the ground.
I've never had much playing time with a Kelly, so I cant tell you much on my opinion on them. If you go to the Jackson forum, you'll get a wide variety of answers on the neck heavy issue (which does appear to be a legitimate concern to SOME people).
The pickups are Duncan Designed on the KE3, I think, and they're not as good as real Seymour Duncans, IMO. The Duncan Designed I've owned in the past weren't quite my thing.
As for the quality, they're great guitars. Better than a RG 570? I think so, but I'm sure others might disagree.
Hearse, a guy that posts here ocassionally owns one and loves his. Maybe he'll see this thread and comment...