I want to be fair to the Winnetu in that they are pretty clear on their website that they are a family-oriented resort, which means families with children, and we were a family of adults with older parents - not quite the profile the resort excels at serving. You can read the various other raves here about the kids programs and services. For us it was disappointing to have every amenity (pools, ping pong table, etc.) occupied with children all the time. It's kind of like vacationing in a very expensive Romper Room.
We stayed in one of the townhouses managed by the resort, across the road and not on the grounds proper. These are nice if somewhat ageing units a short drive or long-ish walk from the main resort building and the activity center. The second and third bedrooms were set up with hard twin beds for children; the only large bed was in the master.
The quality and amenities of these units apparently varies as they are independently owned, and we were disappointed that our unit did not have any beach chairs to take to the beach - we were told we could rent a pair for $25 a day from the resort. I've stayed at many vacation rentals in Hawaii that provide beach chairs, towels, boogie boards, etc. without having to ask or pay extra.
Also the unit had a big ant problem that necessitated having the exterminator come out twice to mitigate. I get that there will be bugs when you're in a house in the woods, but this was a known infestation, and we felt they should have been more proactive about resolving it. We woke up with ants crawling on us, which was not very vacation-like!
As a swimmer, some additional notes: South Beach is lovely for walking along but pretty rough for swimming. I had a better swim at State Beach near Oak Bluffs. Neither of the resort pools are suitable for lap swimming; they're not big enough or marked, and generally there are children and family playing in them.
The gym facility was great, and open late hours; it's at the Activity center, which was fine for us but maybe a bit far if you're staying at the main resort.
We had dinner at Lure, the onsite restaurant, once. The food was good, but not worth the very high prices ($38 entrees). The caliber of service was okay but not what you'd expect at that price point. We found this to be true on other island restaurants as well.
Morning Yoga on the lawn was lovely - very accessible teacher for all levels. We really enjoyed this.
We used the shuttle service to Edgartown a number of times; very handy, the drivers were all terrific. During the day it runs only once an hour (more frequent in the evening).
In summary - I'm giving this a 4-star rating despite a lot of negative feedback, because I think we should have done a bit more homework and found a more suitable accommodation for our needs. We did encounter a few service problems at Winnetu, but overall the place is, I think, as promised on their website, and clearly other travelers with children find it a fantastic place. If you don't have kids or don't want to be around them I'd recommend considering other options when looking at staying on the Vineyard.