We took the trail from Dark Day campground to Vista Point walking, as opposed to mountain biking. it's a nice trail, and as Dark Day implies, under heavy forest cover. One thing worth noting is that despite the fact that you're following the edge of the reservoir, the water is well below the trail, with only 2 fire road access points to the water. There are also a few points where you could scramble down the hillside to the water, but again, no easy or prepared access to the water, and the hillside is fairly steep. Be sure to bring bug repellent - water plus heavy forest cover ensures a heavy crop of mosquitos - our trip in early June was no exception. The mosquitos were intense.
Also, if you're planning on taking a dip in the water (recommended) be sure to bring a swim suit - it's a busy spot for recreational boating, so don't expect to find a private spot to swim au natural.
On the plus side, it's a pretty walk and the views are lovely. The trail is well marked, and well maintained, despite lots of recent erosion due to heavy snow melt. While the overall elevation change is negligible, there is enough up and down in short spurts to keep the walking from getting boring.
On the minus side - it's a reservoir, so expect boat traffic - its a good sized body of water, and there was quite a bit of powerboat noise on a Saturday. If you're looking for quiet and serene, this is probably not for you. You are sharing the trail with mountain bikes, and although the trail is quite well maintained, it is fairly narrow, on a heavy slope and lots of blind corners. Getting out of the way of the bikes can be a bit of a scramble, and the lack of visibility can mean that you and the cyclists don't have a lot of warning that you're going to encounter one another, especially if the sound of a ski boat prevents you from hearing the bikes coming. It's not a bad experience, by any means, but you want to be aware.