I use it on a 4 year old imac, and while it is a little slow, that's to be expected at this point, should be getting a new imac in the next few months, but not sure.
Comparing to CS5 isn't really the same, this is a developing program, then closest thing would be Lightroom, which is also great.
The only thing missing right now editing wise is the built in ability to fiddle with lens distortion with things like a fisheye or even just a wide angle that you want to straighten up the lines. You can get plug-ins that do this though so there's that, and I'm sure it'll be introduced at some point.
The way it connects to flickr and facebook ala iphoto as well as easy mailing etc can sway some people I'm sure, it's nice to cut down on the programs I need on my comp.
I would recommend at least 4GB of RAM to run smoothly, although a recent update has made it work worlds faster. I'd recommend it for sure, and once you get the workflow down it's very nice to use I think, but this is just my opinion.
Best thing you can do is get the 30-day trial of both LR and Aperture and see for yourself, this is only worth it if you shoot in RAW and want to get into heavy editing and tweaking and both programs will take a lot of learning to get the most out of them.