Yes, I have one. For a couple of weeks. It's beautiful. Wow. You hear stories of Apple's build quality, but this thing takes the cake. Shiny, slick, round, heavy yet small - it looks like it came from the future. In a sci-fi film, this would look the part. In my office, it just looks like something from another planet, or the future.
I've seen one demonstrated, running 16 streams of 4k video into Final Cut Pro X (which has been optimised for the Mac Pro) without the Mac Pro's fan whirring any louder. Actually, I couldn't hear the Mac Pro's fan at all over the gentle hum of the room's air conditioning. The 2013 Mac Pro is apparently quieter than a Mac mini. Nor was there any noticeable warmth rising from the top vent of its induction core. And this demo unit was driving two Sharp 4K monitors (Apple doesn't make a 4K monitor yet - the current 27-inch Apple Thunderbolt Display has an impressive, though sub-4K, 2560x1440 resolution).
Really, to do it justice, I should plug my temporary Mac Pro into one of these beauties, but all I have is an old LG at 1680x1050. Sorry. But hey, if you know anyone who wants to lend me a 4K display for a few days of testing, let me know.
Also known as UHD displays, 4k monitors show four times the pixels of the current HD standard of 1080 pixels vertical. The cheapest I've seen in New Zealand is the Asus PQ321Q for $4600 at Computer Lounge. The models I saw in the demonstration were Sharp PN-K321 4K Ultra HD LED monitors which display images at 3840x2160 pixels. These are obviously rated by Apple, since Apple was selling them on the UK Apple Store for £3,499, or around NZ$7028 each.
The Sharp 4K display costs more than the Mac Pro tower, although 4K prices are sure to come down. The Sharp is compatible with the new Mac Pro thanks to DisplayPort (is, Thunderbolt), and it also has HDMI.