I have two of the Mac Pro cylinders - a 6-core for ProTools (HD Native Thunderbolt+Avid MADI+SyncHD) and a 12-core for Logic (MOTU AVB 112d+1248, two MultiDocks, UAD2 Octo Thunderbolt, 4k display, etc.).
They are silent. Zero issues in three years. Raw CPU power is not a massive leap over my previous 12-core silver towers, but I wanted 4k displays, Thunderbolt, HDMI, and all that other good stuff. Cable mess is no worse than before - on my silver towers all slots+bays were full so if I wanted more storage I had to use FireWire, so I just ran up against the limits of what those systems could do. The old rig, with two silver towers with PTHD3 on one and Logic+MOTU PCIe on the other, was housed in a refrigerator-sized soundproof rack. The new rig is two six-space racks with the cylinders on top, and the whole thing fits on my Argosy Dual-15k behind the displays. No more long DVI cables, USB extenders, etc. Much better.
I'd buy them again in a minute - but I am anxious to see what the next revision will be. I'm sure I'll buy into those as well.