I have a Dave's rack on my jeep. I use it all the time for camping, it really helped out with interior storage room.
If it cut gas mileage, I didn't notice, but I do get a lot of wind roar from it. When it is loaded, my top speed is limited to about 65-70 because of drag.
I bought mine used, and shortly after I got it, I had to reinforce the front bars where they mount to the windshield. They had cracked around the welds and were close to breaking in half. I put a sleeve inside each one and had it welded in place. 2 years now, no more problems.
The folding part of the rack is nice, one of the reasons I bought it. I also thought the fact that it flipped back to make a shade was kind of cool. Well, flipping the rack backwards isn't really practical since it is usually loaded, and its pretty heavy.
I paid about $300 for mine, and I would buy it again, but not for the flip feature.
I start what I finish!