Took the plunge and got a ZGemma H7S from WoS with openATV 6.3 pre-installed. I got this as a secondary box so my expectations are not as strict as they would be for a primary unit.
First Impressions:-
The manual makes a good attempt to cover the basics. However the screenshots are too small and dark to be helpful. Naturally, being a man, I didn't read it in any detail

Inside there are two identical power sockets for the HDD. A closer look shows one is 3V and the other the conventional 5V/12V. Confusingly the crude diagram shows them using the 3V socket. I didn't risk trying it.
Setup is the standard openATV wizard for language, timezone, network, initial tuner config and auto scan. The manual also explains things like setting preferred audio etc.
The OS found the HDD without a problem but I didn't find any mention of initialising the HDD in the manual.
CrossEPG and CoolTvGuide plugins will need to be installed and your skin of choice. I never liked the default metrix.
I had backups of my existing bouquets so cheated installing them.
Channel switching is more or less instant as you would expect for the processor and ram size. Picture quality is OK even on crappy Saorview. Not the best I've seen but OK for most viewers.
Downsides:-
The front display includes a very large bright "A" shaped Zgemma logo. I can't find how to disable or dim this although you can dim the rest of the display. I think a piece of tape is required. The rest shows only the time or channel number. There doesn't seem to be an option to show the channel name. That will take getting used to.
Source:- I got mine from WoS for £125GBP inc 2 day shipping. Ordered Thur night with openATV installed. Courier arrived 11am Monday. Image is dated last Friday, so literally hot of the press.
I already had a 120GB SSD knocking around which should be more than enough for normal use.
All in all, you get what you pay for. A budget 4K triple tuner box with reasonable performance.