I am using a Yakima rack on the roof. I have not had very good luck with trunk mounted racks in the past--they have scuffed up the paint on both the car and bike, and I had one loosen up on me on the highway (no bike loss thankfully). If you were hanging a two 15 pound road bikes, it might not be a big deal, but my mountain bikes are too heavy and too expensive to consider on the back of the car. Just my $0.02....
I have yet to quantify the mileage effect of a roof rack without bikes on it, but it doesn't seem to be a huge hit. With the bike on, I've managed a 52.4mpg average for a 400 mile round trip in the mountains, and I can get to local trailheads at around 50mpg (which is probably 5 mpg lower than without a bike). For longer drives without the bike, I pull the rack off which takes about 5 minutes to pull of/put on.
Another nice thing about a roof rack is that you can throw skis, canoes, or a cargo box on it which helps with those long loads that you can't "trunk" in the HCH. However, a roof rack system for two bikes will set you back more in the range of $300 than $150...
Regarding the search, there have been a few HCH specific threads about bike racks. After clicking on the search link (on the peach colored header bar), click on "Advanced search" rather than entering in your search words. This will allow you to select the appropriate sub-forum for your question and get HCH specific results. Edit: Here is the link to the advanced search page:
http://www.greenhybrid.com/discuss/search.php
Commuter car, grocery getter, and summer road tripper--average 10k miles per year.
Winter road trips and ski trips, bad weather commuter and my wife's "daily" driver--expected average 4k miles per year.