I currently use PS CS4 but have a hard time to justify an upgrade to CS5. I edit my photographs: cloning/healing of small artifacts, curves adjustment using layer masks, colorbalance, B&W conversion, cropping/uprezzing, sharpening, blur (just to soften the edges of the masks for smooth transition of the adjustment), lens distortion, stitching of pano's and softproofing. And that all on 16bits file in AdobeRGB or ProfotoRGB color space. That's it. The only new feature that comes close to wanting is content aware fill to do some cloning/healing with more comfort.
Much could be handled by Lightroom/Aperture already but I just like the way local adjustments are done via layer masks and the use of the canon print plugin to circument Adobe's and OSX print dialogs
I suspect that new versions of lightroom&aperture in 2 to 3 years will do everything I need/want at lower costs....so maybe CS4 was my first and last version of PS......