I've used fork-mounted roof racks, strap-on hatchback trunk racks, and 3 different hitch racks.

I had a bad experience with a fork-mounted roof rack and I've never used one since. Silly me, I forgot the bikes were on the roof and I drove into a carport. I cried when I saw both bikes, and the rack on the trunk in my rearview mirror. The bikes were OK, but the rack and the roof of my car, well... let's just say after that, I bought a hitch rack.

Hitch racks that allow the bikes to swing just plain suck. My previous car's bumper is a testament to that. Also, if you carry a bike with a small frame, a non-horizontal top tube, or is a wacky full suspension frame design, forget using the conventional hitch racks that clamp onto the top tube. Again, one hitch rack (of this style) put one of the bike's tires too close to the ground, and directly in line with the tailpipe of my car. Not only did I melt the tire, but upon driving out of a gas station parking lot, the low tire hit the ground and bent the rim.

So, as you can see, I've had somewhat poor luck with bike racks. Nevertheless, even without my misfortune, I didn't like having to remove the front wheel to put the bike on the rack. (What do you do with that yucky, dirty, muddy front tire?) And the last thing you want to do after a long bike ride is have to lift it onto the roof, especially on a vehicle as tall as the Xterra!

The current 2-bike rack I have is an older model of the ones that are currently manufactured by SportWorks . Their racks are a little pricier than most, but loading and unloading the bikes is a snap. It's somewhat similar in design to the one featured on the photo link above. Call or visit some local bike shops to see one in person and get an idea of how it works. The main disadvantage of my rack is that when the bikes are installed, you can't open the liftgate. When the bikes are not installed, the rack folds up, requiring you to fold it down to access the liftgate. On the other hand, the bikes are so easy to remove, it may not prove to be that big a hassle.

PS- Oh yeah, don't even think about doing any serious off-roading if you've got a hitch-mounted bike rack. Your departure angle drops to less than 20 degrees.