I haven't had a ton of time to use it yet, but I am really, really pleased with the 27" iMac so far. The big (literally) win is that amazing screen. It's as bright as the laptop screens, only much bigger and 2560x1440. When the gray start-up screen comes on, the backlighting is even edge to edge. And since it's LED lighting, it also runs a lot cooler.
The migration app worked as expected again, though it more or less ignores additional hard drives. I let it do its thing, then I partitioned the drive (1 gig!) in half and copied over my second drive from the old Mac Pro to the new partition, and named it the same thing. Aperture had no problem finding all of my photos and what not.
I also got a 1.5 TB external drive, which I'll use for video files. It should be fast enough to work from, and if not, I'll just use it for the source material and use the internal drive for the converted ProRes files.
The machines has four USB ports, but I need six. Mouse, keyboard, iPod/phone dock, Shuttle video controller, external drive and card reader. (Machine has an SD card slot, but no CF.) I also might need another port to connect my smart card reader now and then in case I need to login to work. I ordered a cheap hub from NewEgg, as well as a DisplayPort to DVI adapter so I can connect my old 20" Dell monitor as a dedicated video preview.
I'm also contemplating a memory upgrade, since you can never have too much when you're running Parallels with something memory intensive like Photoshop or Aperture. Trying to decide if I want to simply add another 4gig to the iMac (cheap), or get a pair of 4gig sticks for the laptop and rotate those into the iMac (not so cheap). The latter would get me to 8 gigs in both. Oddly enough, they use the same kind of memory, with two slots in the laptop and four in the desktop.
I hope to do some real work on it today. We'll see how that goes!