The first time I restored from time machine, it was pure accident: the logic board on my 2004-ish ppc iBook died, and my new 2008 macbook offered to migrate. I plugged in my firewire time machine drive and was off to the races.
Since then, I upgraded that macbook with an intel 160 gb SSD, and restored from a time capsule back up. No problem. I updated to a retina MacBook, migrated from a now time capsule back up of essentially the same system, and it died. Hardware failure. Proved, with Apple, o. Apple sold me a new one, but had to order what I wanted online. The sold me a temp. Out of paranoia, I built the system back from scratch and just restored my home folder. Then, when the new retina came in (third retina if you're counting), I again rebuilt, and restored just the home folder from time machine.
I have also kept using the old 2008 MacBook. Ultimately, I think I should have not tried switching the backup from the '08 to the retina. In the end, I abandoned that backup and just started new backups for both systems.
Through all that I have never lost a file.
I also have an iMac with a USB time machine disk. I have restored from that disk (after screen and hard drive were replaced under warranty) and the restore hung at "less than a minute remaining." I rebooted, things were weird, I rebooted into single-user mode and ran fsck. Rebooted, all files good, reinstalled 10.8.2 from app store because coreaudiod was acting weird.
As far as I can tell, all files still good.
I also backup everything to CrashPlan (family plan, so worth it), but have never had to go hunting through it to get anything.
Time machine gives me the heebie jeebies but has not failed me despite my worst efforts.