I'm upgrading from ESX Server 2.5.3 to VI3.5. The server and storage upgrades went smoothly. The virtual hardware upgrade on the Windows VM completed quickly and successfully. The virtual hardware upgrade on the Linux (I think Fedora Core 5, but I can't boot into it to verify) has been "In Progress" for 7 hours and won't let me cancel it.
I thought I would post before I give up on it and restart ESX leaving things in who-knows-what state.