I have been playing with Aperture 3 for a while now. The first time I used it (version 3.0) I was not impressed as it crashed and hung and was generally unusable. I recently de-installed it and cleaned up the support files and then re-installed version 3.0.2. This seems to be much more stable and I have imported my entire photo library without issues.
I love the Aperture interface and working in full screen mode. The RAW conversion is great straight out of the box, colours look about right and the exposure curve is smooth and even (unlike Lightroom).
I used to use Aperture 2 before migrating to Lightroom but its been a while so I'm still remembering where all the shortcuts are and familiarising myself with the settings and panels. The adjustments can seem a bit slower than Lightroom sometimes but overall its pretty smooth. There are still a few bugs like the RAW calibration forgetting its settings but I expect these will be sorted out shortly.
Apple need to add a gradient tool, I forgot just how useful it is!
The following D700 image compares Lightroom 3 Beta2 and Aperture 3. The images are cropped the same and I corrected WB in both applications using the dropper tools. I also tried to match the sharpening levels for an easier comparison. Right click to open the link for a larger version in a new window.