The name says it all. Solitude and its neighbor Brighton are under the radar due to there big brethren one canyon over. But that just adds to their attraction. You can find fresh snow here all day long, days after a storm, while Alta/Snowbird have been tracked out for hours or days. The staff is friendly, the price is reasonable, the ski school is great and most importantly the snow is abundant and top notch. I always make sure to spend one day here on my trips to Utah just to ski Honeycomb Canyon. You really can't go wrong skiing/riding here and Brighton, especially if you are here midweek during a storm cycle. The only other places I've been where we felt like we had our own resort were Big Sky and Jackson Hole.
If you have a beginner(s) with your group it doesn't have the best terrain for them, but because it isn't as busy they will feel more comfortable learning here than in a place where people are constantly flying past them. Don't come for the nightlife because it doesn't exist, but if you want that then stay in Park City and just drive over here for the skiing.