I've done a F50, F80, H80. This weekend, I'm doing our production S80 and another H80. As well, we've done "fresh" installs on 3 B80's. No serious problems. I did have a small issue with our first H80. bos.mp and the SSA device drivers did not install, so I had to do some system maintenance to fix it. It took about an hour.
There is an APAR with IBM. They strongly suggest that you unmirror the rootvg first, as there is a condition they cannot yet explain, where a migrated system will install but not boot. The only fix is to reload a previous mksysb and try it again.
Part of our upgrade procedures call for microcode updates as well - so we're doing those also.
BV