I built this system in early 2014 and it has been overall a joy to work/play with. See below for a description of my build and an overview of its performance.
i7 4770K (4.7GHZ @ 1.264v) & 4-way crossfire R9 290 paired with Benq XL2730z eyefinity. Thats 7680 x 1440p eyefinity - about 25% more pixels than UHD (~4K). Since AMD has not yet released single GPUs with 3 x DPs, I have to, for now, stick with DP + DVI,+ DVI. I have OC-ed the DVI up to 120hz and they seem to handle it okay but I have had some detection issues at times when gaming. I suspect the included DVI cables cant handle the MHZ. For now I am running 80 hz @ 1440p eyefinity until the next big AMD GPU upgrade which I hope will include 3 x DP out and drivers for eyefinity freesync.
Most games run at very decent FPS at this resolution. If FPS drops below 50, minor tweaks without sacrificing any noticeable image quality can get it to very playable settings. Dragons age inquisition with tessellation on medium, no MSAA, and everything else on ultra settings nets me about 50-80 FPS. GTA V runs at 55-80 (v-sync on) at max settings minus MSAA and feels very smooth. BF3 runs at > 110 FPS most of the time and BF4 does about 80-100 FPS.
The 4-way crossfire has been overall well done. No major faults which is very surprising since I came from 5850 crossfire which was a big pain back in the day. All GPUs are watercooled with EK blocks and run about 55 C with a pretty good overclock on GPU/RAM of 1167/1375 at + 87mV. Its cooled with 480mm + 420mm + 360mm radiators running in series with two D5 pumps and the CPU block.