I'm using Aperture + PS CS3.
A person earlier posed the question about which is better. Frankly, they address different stages of the workflow BUT your needs might dictate Aperture only vs. both. I.e., if you are a bit more advanced in your need to do post-processing, you may find that you need the strengths of PS. Aperture has correction capabilities but it is not positioned as a PS replacement.
On the other hand, if your needs for PP are minor, then Aperture might easily satisfy that need, period.
Then, on to Aperture itself - I use this in my workflow to simply manage/categorize/tag, etc, etc my work. This is not a strength of PS.
So, for me, they both perform discrete and necessary jobs in my overall workflow.
To the question of Aperture vs. Lightroom...to me its more of a philosophical debate. The market for this kind of tool is currently hot, so expect both camps to jockey continually for position in terms of technical features/functions. E.g., Aperture was first with the Loop and with Stacks (both of which I use constantly); Lightroom now has a somewhat decent loop and I expect will introduce a similar stacking mechanism. So, features/functions aside, it comes down to personal preference on things like the workflow WITHIN the application and it's general ergonomics/look and feel. For me, Aperture (currently) excels, here.
Finally (!) a technical consideration might be speed. I find Aperture very slow on my Mac (2GHz iMac PPC with 2GB RAM). I've read that its more zippy on an Intel Mac and one can expect speed improvements in subsequent releases. However, I don't process thousands of shots a day so I can live with this. Your mileage may vary.
Hope this helps.
Tony.
http://www.flickr.com/photos/welshbaloney