15 feet is high enough for me. For one, I tend to get some wicked vertigo once in a while so it spooks me to go too high. I had one stand that was only 3 feet off the ground since the cover was exceptionally good at that height. I like ladder stands and most of them come at 12, 15 or 16 feet. As long as your setup in a good spot with decent cover, there's not much need to go any higher.

-Mike