Time machine is nothing like Apple's old Backup.app. It's far, far better. Backup is an awful app, that doesn't even work properly (and has caused many people lots of problems). Superduper is still required for bootable backups though, so still useful. Time Machine may suffice for the average user, though.
Definitely worth the upgrade (I got it last week). Spaces, Time Machine, Mail are all worth it.
Some apps still don't work properly (I'm looking at you, Superduper), but most just work as they did with Tiger. Almost flawless.
You'll see a nice little speed increase, certainly with the responsiveness of Spotlight and other UI elements. AutoFS is now multi-threaded, meaning that you won't see the spinning beachball if you eject a network drive non-uniformly.
This is the best review for it:
http://arstechnica.com/reviews/os/mac-os-x-10- 5.ars/