Most of the problems I've seen reported have to do with Nero Vision Express, which I personally refuse to touch after my initial experiences with it. Maybe I'll look into it again later when I've seen that they've fixed it to a useable level.
Nero 6 Burning Rom is not much different from version 5.5 if you ask me. Mainly GUI modifications, and a few minor additions. The most notable is the MiniDVD CD-R format. It still has buggy overburning, but it appears to have resolved the issue of the unusual buffer under run errors, where Nero would be unable to read from the hardrive fast enough to burn faster then 24x or even 12x sometimes! The most annoying thing about that bug is that is inconsistant.
Neither I, nor Nero (it appears) have been able to figure out what is the reason behind the overburning bug, which on some systems will consistantly not allow any overburning what so ever, regardless of the proper settings, and hardware.
So like I said, not perfect, there *are* bugs still, but nothing major. Just don't use Nero Vision Express!