We were finally able to have the stopover in June, so I thought I'd provide some feedback. We are a family of 4.
1) I had been trying for months to see some availability, and there was nothing, so I tried for the last time in May and as if by magic 2 hotels appeared. We chose the Jannah Burj al Sarab (rating of 5*).
2) The booking process was very unclear, and sometimes it showed a price sometimes it showed it free. It seems you needed to add a coupon code to make it free (can't remember the exact code, but it was provided at some point in the process).
3) We arrived at the hotel and they showed us the free room which only had one double bed, they said to add two mattresses would be 250AED total for 2 nights (despite the fact our booking was for 4 people). They also showed us a much bigger room with a double and 2 single beds which was also 250AED. So they had effectively upsold it, although the room was nice and the cost was still quite cheap so it was fine.
4) The hotel was a 3 or 4* standard, but the location was good, nice breakfast, no complaints.
5) Abu Dhabi was kind of boring. The Emirates Palace hotel and the big mosque were interesting, but were only built 20 years ago so there isn't really any historical culture. The corniche was empty of people - we lasted about 4 minutes in the 45C sunshine walking there before having to quickly find a taxi and go to a mall (Abu Dhabi mall - which was OK with a good kids play area). Interesting to have been to Abu Dhabi but no desire to go back.
6) not related to the stopover but the EY onboard entertainment system is rubbish compared to Emirates, they seem have very limited content and nothing I really wanted to watch or play.

So back to Emirates in the future I think.