Hi, I went to Xel-Ha last year when I was in Mexico. We join a tour and so we had a limited time there. I personally thinks it's ok, it doesn't look as great as it seems on the poster and advertising pictures. You can do tube floating and snokeling etc. there. Food is also included, there's a couple of restaurant in there that's not bad. If you like water activity, get in there and get to it, don't go explore around because it's a pretty big area and you will waste all your time.
I would actually recommend this other park close to Xel-Ha call X-caret, it's a way better place.