I'm an iPhoto user. Tried LR and Ap1.5, but I thought they were too expensive at $299 and too hard to use (at least Ap1.5 was) to warrant the buy.
Having played with Ap2.0 since the moment it was announced, I have to say, I've nearly convinced myself to purchase. I can't speak as a Pro, but as an amateur, the interface is great, performance is snappy on my MB2.2, full screen editor with the HUD is fantastic, and even doing things like creating a book is much, much easier. In iPhoto, selecting the page layout in a book (e.g. from 1 pic to 4 on a single page) is somewhat simple, but honestly a little cumbersome and the response a bit slow. In Ap2 it's quick and much easier and intuitive. Additionally, organizing your library is much easier and more flexible in Ap2 than iPhoto.
For instance, in Ap2.0, the ability to mouse over events is now available but it does not FORCE you to use events as a main concept like in iPhoto. Creating folders and projects in Ap2 is much easier than creating folders in iPhoto, and you can organize easier and better in Ap2.
A couple of key tools available in Ap2.0 that aren't in iPhoto: better red-eye (where you can control the level of red eye reduction. w/iPhoto you only get one level), cloning and better spot removal, and stacks (so, if you do, say, 15 pics of the kids for holiday cards, you can stack all 15 behind the best top photo). One thing missing from Ap2 that is in iPhoto is the ability to do simple flags on photos. I use flags to mark pictures from the year that I want to put into a book or calendar. The work-around for me will be to use one start, but I liked the temporary nature of flags in iPhoto.
iPhoto is probably better for beginners, because it limits what you can do w/your photos, and keeps you more out of trouble, but if you are at all an advanced consumer, download the trial of Ap2.
It's running very well on my 4GB 2.2 MB and acceptably well on my 2GB 1.83 Intel iMac (w/ graphics card). A few beach balls on the iMac, but no show stoppers.
