I have a Tepui mounted over the shell of my Tacoma. I picked it up from SoftTopper in Boulder for about $1k if I remember correctly. Reasons I went w/ the Tepui:
-Cheaper than most models out there.
-Takes up a very small footprint on top of the truck. I can still load 5 bikes on the roof rack over the cab while it is on the shell... Yet it opens up to be a little bigger than a queen sized mattress.
-I can open it off the back... Giving me shelter for the tailgate, which serves as our kitchen.
After some use (Probably about 10 nights in the last 8 months) I've discovered:
-The foam mattress it comes with is only a tiny bit softer than a board of plywood.... This is easily remedied by putting a memory foam pad sized for a queen mattress in there.
-Sleeping up there is freakin awesome! So much more comfortable than any other camping situation I've used. And if you park in the trees it's like being in a tree house.
-Getting out to pee is a little difficult in the middle of the night. If you tend to get drunk when you camp, this would be a terrible idea.
-It really hasn't hurt my gas mileage as much as I thought it would. Maybe a 5% reduction if that.
-Keeps me quite warm and quite dry. More so than any other tent/system I've used.
-My girlfriend likes it even more than I do.