I like constant quality, but I have to admit that whoever said that setting it to 20 will give good encodes is out of their mind. Maybe at native res, but most people are watching their 720X480 movies scaled up to 1920X1080 on their tvs. CQ between 13-15 and will get almost perfect encodes. File size will generally flucuate between 2.5 to 4GB per movie tho depending on noise, but if you're looking for long term backups I think it's worth the effort.
I've experimented with constant size extensively, but in my experience it's never as good as CQ at the same file size. For mobile devices sure, but if you're doing serious encodes for HTPC or future proofing, in my experience constant size can't touch the latest CQ. It's still up to the user though, and I respect that people still use constant size, though I can't understand it.