I've now done the upgrade from Intrepid on three machines and here are the results:
1. Work Machine - Uses an Intel 965 Graphics cards, support of which is broken until kernel 2.6.29 / 30 according to the forums, and won't be properly supported until karmic. Fucking annoying, since by 'broken' I mean "utterly non performant".
2. Home Laptop. Ran just fine, but apt decided that I didn't want the restricted packages installing on the upgrade, so my atheros wifi card was out of action until I could get the .deb from somewhere else. Annoyingly, the madwifi drivers seem to kill whatever ubuntu do to NetworkManager so I'm stuck using the propietary, even though they've been in the kernel since 2.6.25.
3. Home Desktop. This has killed X. Killed it good. At the moment I've no idea how it was managed, but it was...
I love ubuntu, but this upgrade has gone horribly wrong for me on two out of three installs, and 'smoothly' on none of them.