I started out with Carefresh but switched to YN because of dust and odor issues (husband is very allergic to just about everything).
They didn't like the YN at all, they push it all up into a large pile in the middle of the cage and sleep in the bare corners. I think they prefered the softer texture of the carefresh.
YN is expensive, I buy the cat litter varity (unscented softer texture) instead of the type for small animals as the price difference is considerable! ($18 for a large bag of cat litter or $15 for a small bag of small animal litter). I was still using SO MUCH of it in the martins/tommy cages (it's a tommy but everyone calls it a martin so whatever
), I'd go through about 3 large bags a month which was getting really expensive.
They stayed on the YN for about a year, even though they didn't like it all that much and they would constantly push it all up into the large pile of litter in the middle of the cage.
Now I'm doing a mixture of litter boxes, YN and terrycloth towels. I put the towels down in the cage and put a few litter boxes full of YN in the corners and they took it right away! I was really impressed!! They started using the litter boxes almost instantly. I only had to pick up poops and put them into the litter box a few times that first day. Sometimes they get lazy and still poop on the top shelf, but if I stick a litter box up there they just knock it over, plus they all sleep up there so there isn't room for a litter box.
Also, their litter boxes seem to double as the dinner table for them. I can't get them to stop going to the litter box to eat! lol! They pick up their food from their dishes and run to the litter boxes to eat. Oi! Oh well, rats will be rats I s'pose!