I've been there twice and thought it was awesome. There definitely aren't miles of good flow like Glamis, but from the lake west to Devil's Dune is friggin' awesome.
First time we did two weeks the end of July beginning of August 2012. Two day drive up, two day drive down.So 10 days at Sand Hills. We would dune for a couple of days, then head out somewhere to sight see. Yellowstone, Jackson Hole, Bear World and Ice Caves... This broke things up nicely so the wives didn't get burned out on duning. There were definitely some hot days, but it was only bad in the middle of the day, it cooled down nicely at night.
Second trip was in 2022. We went in early June right after the kids got out of school. Same drive times, but we stayed at a buddies house for a night or two on the way up and did two nights at Lava Hot Springs on the way home. This left us with 5 days @ St. Anthony's. Given that it was early June it hadn't warmed up yet and we lost a day or two to wind. Kind of a bummer, but still a good trip.
I didn't really notice the decline at Sand Hills, but the park was kind of a mess with mud puddles due to some pretty big rains right before we arrived. Will probably try Idaho Dunes for the next trip. Hopefully next year.