I also thought Time Machine was great when I got my first mac laptop last year, but since then I have found at least three things that really bug me.
1) Time machine backups can only be accessed from a Mac. Of course, I only realized this when I had my laptop in for a repair, when it turned out that although my files were safely backed up, they were completely inaccessible from my PC desktop.
2) Time machine keeps making incremental backups until it completely fills up the destination drive. Only then will it start rewriting the old incremental backups. There are reports on various websites that you can limit the size of the backup volume, but I haven't been able to get it to work. In the end I had to delete my backups, set up a new, clean partition on my external hard disk, and let Time machine use that.
3) Time machine doesn't work too well for backing up your local email mailbox. Pretty much every time your local inbox receives a new email, Time machine realizes that the mbox file has changed and runs a backup. Having a 2 GB mbox file backed up ten times a day is a drag - it clogs up the wifi connection and keeps the HD spinning.
Now I have Time machine back up only my OS X system/applications, and only after I install someting. I have other software back up my personal files. That way my files are backed up in PC readable format, and I have more control over how and when it's done. Time machine is slick, but has issues, ties you to the Mac platform, and it doesn't give you enough control.