My wife has, more than a few times, nearly pitched her WM5 device against the wall (which wouldn't really solve the problem, would it?) because of the exact same things you mentioned. Synching has always been problematic, but I could always get it to work eventually until now. The worst part is the overwriting or deleting of what are clearly newer files. I can't explain it and I can't fix it. On my WM6 beast, I gave up on synching e-mail entirely. I just takes too long and sometimes just goes off into la-la land forever (until I manually stop it).
I haven't had any stability issues. In fact, I think it's overall more stable for us, but I do get annoyed at the screen real-estate eaten up by those two soft-buttons at the bottom and the snooze options that just disappeared in the transition from 2003SE to WM5.
HTC HD2 US (unlocked) + 16GB micro SDHC (in holding)
HTC Evo + 16GB micro SDHC (in use)