I use M mode exclusively on my 600 EX-RT and I also shoot in M mode. I've memorized settings that work under various lighting conditions and at various distances and I'm really pleased with the results that I get. I really like the flexibility and control that comes with shooting in M mode.
I switched from APS-C to full frame a couple of years ago and have never looked back. I do a lot of closeup photography and for that kind of work low noise is an absolute must. I'm skeptical that even a vastly improved 7D will give me the kind of image quality that I get with my 5Diiis.