I am planning on pulling the trigger on one of these in the next month or two:
http://www.jamesbaroudusa.com/discoveryspaceevolution.php
My negatives about the fold over type RTTs are very minimal. First off, they are huge up on top of the truck! I could feel mine up there especially with a foam mattress and sleeping bags, pillows, blanket. It was not aerodynamic and i really could sense a loss in gas milage and performance. Then there is the time it takes to set up and take down. Now that's being picky because it doesn't take any longer to set up then a regular tent. But, when it's wet or if you are breaking down first thing in the morning, the dew is on the tent and you hate to fold it all up wet. Lastly, in the east on tight trails the travel bag would get snagged by branches and rip.
So, after seeing several hard shell RTTs, i'm sold on one. They pop up in seconds and fold down in seconds. Even if they are wet in the morning, a quick wipe of a towel and they are ready to fold down. You can leave all your gear in them and they are lighter and more aerodynamic.
With all that said, i will miss the changing room when i'm camped in the same spot for days. It was like having a private room for changing, cooking, taking a poop or anything.