I have one of the racks that holds the wheels in hoops and has a hook that pulls down on the top tube. It's a little cumbersome to get into the hitch, and more cumbersome to put the bikes on. but it does hold them quite stable. they don't move much, and thus can't slam into each other or the car. i have a new Civic and didn't want to get the trunk rack, or roof rack, so i had a hitch installed (the expensive route, i know) but it is nice to have a rack that works on my wife's SUV too.
I did have some problems with the bolts that clamp on the hooks being very very stiff and difficult to turn, and when you had it as tight as you could get it, and it felt very tight, after a couple miles of driving the hook would pop loose and let go of the bike so i started bungee cording the bikes tightly together. I took apart the clamps and saturated them with WD40 and now they are smooth, easy, and secure. they don't pop loose at all anymore. my dad has the standard hitch rack where the bikes just sit on 2 bars and a velcro strap holds the top tube. his is sooo much faster, but the bikes sway and hit each other and it just doesn't seem as secure. other than being a little slow to get set up, my only complaint about my hitch rack is that it mounts onto the hitch with a bolt rather than a cotter pin. i understand cotter pins allow the rack to wiggle and shake more but the bolt isn't as secure as a locking pin would be. and it is a very long bolt so it takes a while to install/uninstall. also the nut for the bolt is welded inside the hitch rack neck, so using a cotter pin isn't even an option unless i break off the nut inside, in which case the bolt would no longer be an option overall i like the rack though.