I have been using Aperture 3 since it came out and I just upgraded to an iMac 27" i7 with 12 gigs of RAM and I am unimpressed with the way Aperture runs on it. It was so bad I went back to the Apple store and checked out another machine to see if it was faster. The only had an i3 on display and it was about as fast as my i7.
I've read that the i5s are better machines and so I may have over bought. Anyway what I found was that after I first started up Aperture and then quit it and restarted it, it would come up in about 2 seconds. Editing photos was OK in the beginning but after a while it would slow down. Going to full screen sometimes was like watching a slow motion event (and I'm thinking I just paid a fortune for this and it should be like a rocket.) Using the spot tool again would work fine for a while and then I would click 4 or 5 spots and there would be white circles where the spots were. Then all of a sudden they would complete the command.
I also had a copy of Lightroom 2 which I upgraded to 3 and it is wonderful. It loads photos much faster than Aperture does. To really speed it up though ( I think this would word with Aperture) is to copy the card to your drive and then import them. Lightroom is a lot faster at editing photos in all ways. So using it on a daily basis is a much less frustrating event.
Both programs have strengths and weaknesses. Aperture beats the hell out of Lightroom in making slideshows, web pages and making books. But the basic editing is really crappy and that's what photographers need is a program that you can keep track of your stuff in a well organized way and do a lot of you editing needs painlessly. I would say that either program but especially Lightroom replaces what most photogs need Photoshop for. It takes a long time to get efficient with PS before you could work as quickly as you can with these two programs.
Oh and Lightroom prints a whole lot better than Aperture does.