We just got 12 x HP 3800-48G-PoE+-4SFP+'s installed last month in one of our offices, originally as standalone switches linked together via ethernet and once the stacking modules turned up (supply for these is still scarce in the UK it seems) we got them stacked last weekend in to two lots of 6 linked via 2 x fibre SFP's.
Installation of the 2nd PSU's and stack modules was very easy. Configuration and stacking of these was also easy, no unexpected issues there. Performance has been good, the switches run quiet and the PoE worked well.
However, we've been having a few issues in the past week with a section of our PoE powered VoIP phones resetting randomly, and I've just discovered in the logs today that it looks like the Commander switch in the stack has crashed/rebooted around the time that the phones went out. Make of that what you will. I'll need to do some more digging.