Macintouch has some good early reports on Aperture. Regrettably it doesn't sound like I'll be switching just yet.
Aperture's system demands are staggering. The paranoid might suspect it was designed to suck cycles, irregardless of cost/benefit measures. Images are stored in a database, they cannot be accessed separately. I'd put the probability of significant database corruption in early versions of Aperture at 99%. Metadata is locked into Aperture, there are few if any facilities for export. UI elements are odd and sound half-baked.
I'll definitely wait for version 1.1. I want to see how dedicated Apple will be to the "lock in" philosophy. (Lock-in is perennial Apple vice. One bite and you're their's for eternity ...)
Update: one of the oddest defects of Aperture is that you can't change image dates in some file formats. Very annoying!
Update 12/2: while waiting for 1.1, I'll read the manual.