I'm having a hard time deciding. I've had trouble with Backup (periodically decides not to recognize my .mac password, resulting in scheduled backups not being run) and email reliability (frequent downtime, plus very often my incoming or outgoing messages will be delayed by as much as 6 hours!).
In the end I probably will end up renewing, just because it would be a pain to switch email addresses and not have the convenience of iDisk when I need to transfer files around. But $100 is really steep for that, I think, especially considering the features are not 100% reliable. If it were half the price, I'd renew without even thinking twice.