If you're used to Open Office, and use a lot of it's features, then KOffice will largely be a dissapointment. It's document filters are no-where near as good or as varied. For most things, Open Office beats KOffice hands down. The only things that KOffice is better at are:
Desktop Integration: Set up a KDE theme and KOffice will play along. Open Office is an ugly dark gray all the time.
Startup Time: Particularly an issue with older versions of Open Office, KOffice doesn't have the start-up issues that it does. Though, having just compiled my own OO.o Beta2, the start-up is a lot better...
That said, I am still looking forward to KOffice 1.3. Not so much now that Kexi (the new database front-end) won't be ready in time, but improvements are always welcome.