I've had the best results with CPC. CPM has yet to have converted to a level anywhere near profitable for me.
so far cpc with adsense, then cpm with fastclick. fastclick is a special case though because they have great rates. other cpm networks probably might not do so well.
I use fastclick as a second tier cpm network. Burst also goes as less frequent second tier (usually third or fourth tier depending on the creative size). I use a mix of Casale and adsense as a second tier for my medium rectangles since adsense gives me mainly CPM for that size. Burst was performing horribly in that size so I've taken them off completely. I'm sure you guys can guess what my first tier CPM only network is. Hint: I haven't mentioned them in this post and they're the best ad network. EDIT: I'd have to say that I prefer CPM based advertisements because I like to have nice aesthetically pleasing graphical ads instead of text ads which are sleazy/unprofessional/amateurish looking.
I agree, cpm ads usually look better usually, and at higher traffic, they are probably better in the long run, I just use everything