I have two types of Quartersheets - and they are used primarily for colder weather riding.
The kind that goes under the saddle: http://www.doversaddlery.com/product...&ids=986065587
Is used for hacking primarily. I dislike this one personally, that's why I have mine for sale.
Now the one I love and used allot over the winter/colder weather is this one: Riders International Fleece Exercise Rug - Dover Saddlery.
I like this one because - it has more than one use. It keeps me warm, and my horses hind quarters warm. I use it for trail riding - keeps the cold wind off my legs and my horse.
I also use it when I warm up in the indoor during the winter months. It helps warm up my horses hind end, and it keeps my legs warm too :) Then when we are warmed up, I'll take it off for jumping sessions.
Then when we are done our session, I will toss it back on for the cool off.
Also, during the winter schooing hunter/jumper shows - I will put it on over my hoss during warm up, and then take it off for our rounds, and toss it back on his back end after our rounds - keeps his muscles warm.