I imported by reference a 2300 image iPhoto Library -- all JPEG. After importing I practiced browsing it, and I tested the filmstrip "Preview" browser (tap P to toggle). Preview browsing is extremely fast, at least with JPEG originals. Scrolling the entire set of 2300 images was not as fast is iPhoto but was very acceptable for my purposes. In practice I almost never view that many items at once.
The import worked well, as before slideshows, photo books, calendars, etc are not imported. If you want to switch your iPhoto images to Aperture you need to save these as PDFs. Since iPhoto and Aperture count images differently it's tough to know if all images were truly imported. Movies will NOT be imported from iPhoto, in the past Aperture didn't warn they'd been left behind.
As with earlier versions Aperture imports Smart Albums as "dumb" albums. Aperture's "smart albums" are as dim as iPhoto's: you can't use a Smart Album as an input to a filter (no nested queries in other words). iTunes is more powerful.
I expect I'll buy the product in a week or two.