Originally I used DPP because my old PC was too weak to handle CS2. I didn't have a calibrated monitor at the time - ok, I used Adobe Gamma, and used a 7 year old CRT. I bought a new PC and NEC20WMGX2 monitor, calibrated it and realized my photos looked like crap. With my new monitor and rig, I switched to ACR/CS2 and I ditched DPP because I liked the idea of a RAW converter integrated with PS. With DPP I was still going to PS for angled cropping, highlight/shadows, saturation, resizing and sharpening. I figured CS2 had a better workflow as it was already integrated into PS. Plus, I wasn't too happy with the results I had from DPP (which in retrospect was NOT DPP's fault but rather due to working in a unmanaged color environment and old uncalibrated monitor). I have upgraded to CS3 and really like it's UI compared to DPP.
I was surprised to see so many people using DPP on this thread. So after typing my long previous post extolling the virtues of CS3, I decided to play around with the upgrade to DPP and I have to say I am quite impressed. The UI is still crappy and cumbersome, the degree of control over certain aspects is somewhat limited compared to ACR but for some reason the colours are quite amazing. I really am going to give DPP another look. It really runs counter-intuitive that DPP, which offers control as compared to ACR 4.1, can still churn out sometimes better results than ACR 4.1
Maybe I'll be back to the ol' workflow of DPP and then PS. DPP really does give a better color rendition than ACR4.1. Maybe I'll use ACR strictly for important shots where I've bungled the exposure and need it's highlight recovery/shadow detail. I am fairly confident that ACR does beat DPP/PS combo for highlights/shadows.
Maybe there is greater merit to the argument that Canon knows its CR2 files better than Adobe and that help DPP process Canon RAWs better?