I go next week for a fortnight so I will be back before you go, so I will report back to you.
I think it is a good sign that your friend has been and enjoyed it, as you will sort of know what your friends expectations will have been and that they are being honest, where you have no idea of the expectations of strangers. As you said if you are used to 5 star in the Caribbean then 3 star in Spain will be much different.
Like you, we go to all inclusive in Spain most years so I feel like I know what to expect, there are always some groups who you would try to avoid, kids that are running wild etc, but I do think it is generally mostly people who are just trying to have a nice family holiday. I've also been to Butlins and had a good time and the reviews for there are shocking! 
Also (trying to convince myself here!) I think that most people don't tend to write reviews if they have had a nice, unremarkable holiday, but will write a review if they have something bad happen, i know this is the case with me. Looking at trip advisor there are still many more people who have really enjoyed it and had no issues than those who have hated it, became ill, had a bad experience etc.
I think you are right in that most of the illness seems to be in families with young children, DD is 12 so hoping we will be ok, no need for us to be in children's pools or using the kids buffet, so fingers crossed.
If it is really bad I guess we can just go to the beach and use the waterpark more often.
DP who is Mr Optimist is saying that there is no point in worrying unless we have something to worry about, which I guess is true. 
I am wondering how busy it will be next week, I know the Scottish schools have broken up already (we are in Scotland) but I don't think the English schools have?!