Been playing with it (finally) this evening.
I can definitely see how the vibration would bother some. Not a deal-breaker for me, but if you're having any doubts, I'd highly suggest checking it out before buying. The maximum volume on this one seems to be a good bit higher coming from the 4. Thankfully, I haven't heard any sound defects, even when the pad is vibrating at higher volume.
Very very snappy. I've had an iPad 2, a 4, and now this one. In older models, it seemed like the more content I loaded onto the drive, the slower the iPad would get.. but no evidence of this whatsoever. I currently have about 100GB of music/tv/movies on this one, and it's still as fast as when it was near-empty. I'm not too demanding on Safari, usually keeping a maximum of 4 or 5 tabs open at any given time; thus far, I've had zero tab refresh instances, even on photo-heavy pages.
I had also read a report on an Apple forum about how some folks were getting iPads with screen defects, but I'm not seeing any evidence on the one I have. No dead pixels, no dark spots or distortions.
I'd avoid the 16GB model. Not much room for holding any photos, apps data, etc. I'm not really sure why they'd sell such a model; its longevity would be limited.