Hi,
I'll start by saying I've had roof racks and my current rear rack. I think roof racks are more functional AS RACKS. However, I chose the rear rack because I didn't want to deal with marring of the paint.
Here's my rack:
It can be removed in seconds by unlocking a single lock and removing ONE pin. Besides carrying what you see above, I carried a 6250-watt generator just before Hurricane Isabel. Then I carried a 5.5-hp Lawn Vacuum just after Isabel.
The pitfalls:
1) Scrapes in steep driveways
(might not be an issue with dedicated bike racks)
2) Soot coverage. Short trips are not likely a problem. But long trips yield a coat of soot on the parts closest to the tailpipe.
3) A roof rack has more surface area. Therefore, a roof rack can carry two bikes AND a canopy filled with baggage.
The benefits:
1) Super-quick removal and installation, including the lock.
2) No high lifting - Imagine trying to get a generator or lawnmower to the roof alone. /images/graemlins/wink.gif
3) So, you mountain bike? I had enough of muddy bikes dripping on my car! My bikes drip on the pavement now.
4) Although my mileage suffers compared to no load at all, I can still stay in the low-to-mid 40s. I'm not sure what kind of mileage I'd get with a roof rack.
Some details regarding my rack can be found
here. Scroll toward the bottom.
Good Luck with your decision...
Scott