For the size of catalog I have, how I work and being that they're all RAW files, Aperture is a dog for me in many ways. I can not waste 20 minutes at a time staring at beach balls in Aperture just because I have other apps open at the same time. Using selective brushes pretty much brings things to a halt, as does simply importing a new catalog.
I like its GUI, and it has some neat tools, but those things would never replace what I can do in Lightroom 2. A lot of people will argue that LR doesn't have the same tools as Aperture, but after examining said features, I found that it isn't true. It's just that certain features aren't named the same way, so it can be confusing.
Whether it be with my 13" base model MacBook Pro, or my earlier 15" MBP with 4 gigs of RAM, Aperture runs horribly while Lightroom 2 is just the opposite. Interestingly, Aperture was the first DAM I tried when I bought my first MBP, and it was exactly the same. Slow to unresponsive at times. And I'm not the only person who has made this observation. You'll likely hear none of the complaining here though, since this IS an Apple centric forum. You'll get more unbiased reviews and comparisons on some photography forums. Since you're a Canon shooter, I'd suggest visiting POTN.
Though you can find the same reviews on my photography forum which is the nikon cafe, and we actually have a lot of Canon shooters as well as all sorts of film shooters.
Good luck.
