Have had two S5 running flawlessly for three weeks and today experienced an issue (batch four, OE file system, CGminer 4.9.0, 306.25 MHz clock). One of the two appeared to have frozen. Perhaps half a day elapsed while both controller and Hash appeared stuck. Discovered the problem by observing lower than expected Hash on Eligius and traced it back to the specific S5 by addressing it, whereupon it displayed the miner status page (without my logging in or selecting it), but was unresponsive to GUI selections. I shut down the PSU for sixty seconds, fired it up again, and back to normal. It now once again appears fine.
Noticed today my Westhash account was down ~2TH/s. Did a visual on the miners and all appeared fine. Tried addressing the two S5s and neither responded. Power cycled both, checked their status (as well as the S3s), and all was back to normal. The temps are 42/44, 46/46 and neither has been o/c'd (at least, not since Bitmain shipped them.) The power supplies are CX750M with all four PCI-E connectors populated, the file system is the Jan 7 version, CGminer is 4.9.0, and the clock is 306.25MHz.
PS. Took a look around and it appears Batch 4 is sold out and I see no mention yet of a Batch 5.