I use the syncing, and though it's really usefull, it's unreliable and often slow. Seems the UK (or Europe) service is poor compared with N.America. There's often one or more sync items that won't work first time and so need user intervention so automatic syncing is not a practicality. Occasionaly .mac is completely uncontactable. It's the same story with both my ISP's so I can't blame them.
Why do I stay? Nice short email addresses and a hope that Apple will sort it out eventually...
If I wasn't already member, and locked into email addresses and webspace (by my lazyness), I wouldn't bother joining.