Aperture 1.5 was very usable on my ProBook (2 GHz, 2 GB RAM), although Aperture 2.0 and 2.1 are a significant improvement in terms of speed, I dare call them fast even. Aperture's interface has matured quite a bit with this upgrade. Pictures are exported in the background now and you can continue your work, the controls are responsive and nice on my machine. Aperture 2 gives me the feeling that it's really a work
flow.
I tried Lightroom 1 and 2 Beta, their interface doesn't compute with me
(I find it ugly and bloated, but that's obviously a matter of taste.)
Look up the usual thread, but whether someone prefers Aperture or Lightroom is in large part determined by the way they work. If you're not working in a linear manner (import > sort > correct/develop, etc.), then you will likely prefer Aperture. Lightroom locks you in its workflow and if you happen to agree with it (or you want to go along), then it's (apparently) working great for people. If you don't, then it constantly gets in your way.
There are very few features that distinguish one from the other. Aperture 2.1 has introduced Dodge & Burn, although it seems to me, it's a bit of an example for people who develop plugins. It works well, but (obviously) cannot compete with Photoshop or a real photo editor. Lightroom will introduce similar tools with version 2.0 which is currently in beta). The advantage of Lightroom 2's tools is that Dodge & Burn is non-destructive whereas Aperture renders the raw file into a tiff and applies Dodge & Burn to it.
Aperture 2 allows companies to make plugins and there are many in the works (e. g. noise ninja and various filters). I'm sure whatever Apple hasn't supplied us with will be added via some plugin. AFAIK Lightroom does not offer a plugin architecture.
People also frequently complain about the omission of curves in Aperture. I'd like to have them, but they're not an `I can't live without it' feature. Apple offers various sliders with which you can achieve similar effects.
Aperture beats Lightroom with books. The book printing service (despite its recent server hickups) delivers astonishing quality. Brought tears to my grandmother's eyes, literally, and I've seen her cry only once before. I've made a wedding album and it's just amazing. Plus, even though I've discovered some bugs, 3 of them have been fixed in the 2.1 update! The improvements in the book editor were one of the strongest reasons for me to upgrade.
Aperture can also handle transfer of pictures to flickr via plugins, for instance. I'm not sure whether this is possible with Lightroom.
It seems that `smart albums' will be added in Lightroom 2, Aperture has had them from the beginning.