Very happy, with some considerations:
Mine initially had a bunch of random restarts. Both in Yosemite, and Windows 8.1
For whatever reason, those have vanished on the bootcamp side, and are much less frequent on the Yosemite side.
That said, I still get an occasional unexplained reboot, and the screen saying to press any key or wait for a restart.
I've not seen that happen on my 2011 iMac.
Not sure if my troubles can be traced to either my USB3 hub, or one of the external drives on the system, but I ended up building a USB3 RAID disc I had been planning anyway, and intend to remove the other drives, and possibly the USB3 hub.
Otherwise, it's fast, beautiful, and does everything I ask of it. I play some Steam games (mostly in Windows, which is the reason for bootcamp) like Dark Souls, and Torchlight, among some others, and it handles them beautifully.
Mostly Lightroom stuff in addition to regular computer tasks on the Yosemite side
Mine's the i7, 295, with 3TB fusion and 24mb RAM