I don't rely on .Mac's Backup, but I tinker with it from time to time. It's never seemed to be the killer app to fit all my needs, but others in the Mac web who are more passionate and knowledgeable about backup software seem to be quite upset with it. In particular, Jonathan 'Wolf' Rentzsch, Michael McCracken and Erik Barzeski more or less echo the same opinion: Backup needs help, and a lot of it.
All three of these notable Mac personalities have penned horror stories and useful overviews of what is so wrong with the little .Mac app that (someday) could, and I found them with a recent Apple kbase article John Gruber linked concerning a silly restriction of working with Backup 3 files.
After reading through these, I think I might leave my Backup tinkering as just that, and rely more on ChronoSync since it does machine syncing in addition to straight backup. Though the fact that I haven't seen ChronoSync written up as good or bad in any of the recent reviews makes me curious. Has anyone heard anything?