I've got an older Thule 'prong' rack that I got locally on the Buy, Swap and Sell page for $20 or so.
Works fine for my bikes (XL frames) but no good for the wifes Small framed bikes as the top tube isn't long enough to hang neatly on the prongs.
In the past I've used fork mount roof racks, and much prefer those.
One last problem... I flip my towbar over so the tow ball is at the highest setting as with 3 bikes on the back and 4 in the car, I found on steep driveway to road transitions that it bottomed out and knocked a bike loose on the rubber frame holders.