Actually I am using a new imac and getting used to it (from Ubuntu 9.04). I would give Ubuntu the edge in many areas. Gnome, the gui, cut and paste, etc are all better with Ubuntu. Ubuntu is much faster than OSX (I must admit I have a quad core 8mb ram workstation for Ubuntu). Apple has excellent hardware. The display is superb. Access to software is better (MS Office, Photoshop, etc). However, if you don't need the commercial software, then things get real interesting.
I booted up the 24" imac with the live Ubuntu amd64 cd and everything worked (sound, airport, nvidia video, etc). I must admit I am really tempted to install Ubuntu on the mac.