I've had no real issues in a 2H/2J system, connected via HIC.
Outside of Hopper-to-Hopper sharing, both hoppers have been very, very solid. I haven't had any sort of lock up or reboot with general viewing in months. Very early on there were a couple of spontaneous issues, but none for a looong time. Never had morning lockups, connection issues, or any of the other common complaints. The one ongoing issue is occasionally hopper will not connect with one of my EHDs(I have two) after the nightly reboot, maybe one day a week or so(maybe less), unplugging/replugging the EHD will fix it.
I do have a recipe that will cause issues when using Hopper-to-Hopper sharing, watching two live/nearly live PTAT events from the remote Hopper and pausing/swapping back and forth between them. Hopper seems to lose track of what it should be displaying at times, sometimes mixed video/audio, sometimes black screen. Almost always can now recover it without a reboot though.
Never had a moca-wired Joey connection issue. I've done extensive testing of cat5 and wireless, running several weeks on each. Never had a cat5 wired Joey connection issue. Only Joey connection issue was re-aquiring the wireless signal if the router reboots, considering it's not officially supported I can forgive it. Currently back to all moca-wired operation now, the network tests were more from curiosity than need.
Only Joey issue I recall is a lock up twice while using the browse banners. Locked hard and required a front panel reset. It's been a while now, but I wouldn't claim it as fixed, the two times before were quite a while apart. Also, occasionally Joey will not come out of standby for an IR remote.
Outside of H2H sharing, Hopper/Joey overall have been as stable as my 722/722k's were, especially if you factor out the first two or three weeks.