I keep seeing ads for bike racks where the seller writes something along the lines of, "for sale, due to lack of use," or something similar. As I'm about to buy one I've started wondering what it is about racks falling into disuse? Do people just end up preferring to sling their bikes in the backs of their cars?

What feels like a good idea at the time becomes Kid's / Mrs mastering excuses?

I gave up on the strap on rack we had and started slinging the bikes in the car – but the tow bar mounted one we got when we got new clean cars is excellent.
Quicker to put on, actually legal and very secure.

+1 for towbar mounted
http://www.halfords.com/webapp/wcs/stores/servlet/product_storeId_10001_catalogId_10151_productId_172555_langId_-1_categoryId_165515
If you have a tall car roof mounted ones are a bit of a reach but otherwise OK
http://www.halfords.com/webapp/wcs/stores/servlet/product_storeId_10001_catalogId_10151_productId_170000_langId_-1_categoryId_165515
strapped to the tailgate is not good

I'll keep the towbar ones in mind but sheet of plastic in the back is still looking good at the moment due to the lack of enthusiasm for strap-ons.

I owned a rack that strapped on the back for 45 minutes before it was in it's box and back to the shop.

Tow Bar has been the best bike rack I've had and still use, and before that roof racks were ok, but the tow bar rack and one that tilts is an excellent bike transport solution for a car if you don't want to bung the bike in the car.

Mine's a slidey towbar mount – possibly better than a tilting one (though you pay for the privilege)

check out Roofbox.co.uk for a good range of stuff. I've got the Atera and it's good kit (did the run to Les Gets with it this year), and folds up small (big hiont – that is really worth it) but there are much cheaper options that work nearly as well.

If you go for one and you have a choice on electrics the 13-pin Euro style plug or the twin 7-pin UK style plugs are needed if you want all the lights to work.