We have been car topping our Kayaks and boats for several years now (have not lost any yet).
We have a camper that attaches to our hitch so we cannot use a boat trailer, also at our homes both in Sarasota and Key West the neighborhoods are restricted so we would have no place to store a trailer. Basically this leaves car topping, or the alternative is " no boat for Bob".
We travel all over the country with our camper and on many occations I have multiple Kayaks up there (all Hobies of course). We have a GMC Yukon Denali, the roof rack is rated for 250 lbs, however when I lash the TI down we are tying to the frame of the rack at the edges (which I assume is much stronger).
We also have a T-bar (goal post) that hooks to the bumper hitch that I really like, when not hauling boats we just remove it.
I'm thinking we have around 60,000 miles on the vehicles hauling with boats on the roof with no adverse effects.
I'm not a big guy and have never had any difficulty getting the boats up and down all by myself. Basically you only ever need to lift half the weight of the boat (around 45-50 lbs), which is easily done. I typically pull the boat up behind the car, then hoist the front only onto the T-bar (I bungy a rug over the rear of the TI to keep it from getting scratched on the ground). I made a pool noodle covered in a canvas sheath (actually an old Hobie sail bag). I place the noodle on the roof. I then lift the back of the boat and slide it forward, it rolls on the noodle as it moves forward. I have no Yakiman, Thule, or Malone Kayak racks of any kind on the car just the noodle, and the cars normal cross bars (which the boat doesn't touch). I then slide the AMA's along side the hull (one on each side), then strap the whole works down with 1 inch cargo straps. One in front, and one in back of the roof rack (actually tying to the frame of the roof rack), then one more strap tying the rear of the boat to the Tbar. The sail is tucket between the hull and one AMA. The boat does not move at all and is very secure (even in high winds). With my old Malone V wing racks the boat would rock back and forth when you went over railroad tracks, it no longer does that since I took the Malone racks off.
If I'm going to have the boat on the roof for a long period of time I also put a couple noodles into the two grooves ( the two grooves from front to back on the hull bottom), this helps keep the hull from deforming in the hot Florida sun. We sometimes have the boat on the roof for weeks at a time (secured with heavy duty motorcycle locks).
Here is our car with the Kayak on the roof.
Yes fuel mileage does suffer a little, but that depends on speed I think. On a recent trip up from Key west to Sarasota, I was getting 22 mpg when going northward (with the wind that day, and a little slower up US 1). Then when I turned west (into the wind), and up to 75 mph across Alligator alley the fuel mileage went down to 13 mpg compared to normal 18mpg on the highway (at that speed), If we would of had the trailer in Tow the mileage would have been around 9mpg at best.
Hope this helps