1.) Did you like it? (Why or why not and if not, what did you end up using instead?)
I currently use it and have used it for, I don't know, about a year. I use it in my 4' cages. I like it. It's relatively easy to use once you figure out the best way to line the cage, it cleans up easy, it provides a 3D surface and I noticed less spastic behavior from my boas the moment I introduced them to it. They weren't flailing around trying to grip onto something when I put them in the cage (used newspaper and dimpled craft paper before). They also crawl around more often and at all times, have a higher body profile because they're not spreading out their ribs for traction.
My tubs still get dimpled craft paper. I will not use paper towels because I've seen animals swallow paper towels and die as a result.

I've also seen animals swallow it and live, but I'm not willing to take the risk.

Neither of those pics are mine, but I asked the owners who said I could repost them.
2.) What age/size is appropriate to use this for?
I would think any size is ok as long as you can fit it in the tub. I also run it up the side a little to avoid poop in the corner and the stuff is slightly rigid, so bending it prior to putting it in will help with lining the cage or tub IMMENSELY.
3.) How cost effective are corrugated cardboard sheets compared to aspen? In general, is it more or less expensive than aspen.
I've never used aspen, so I don't have any direct experience. I haven't bothered to find it locally and I order mine from Uline.com. I get a 30" wide roll that's 250' long shipped for about 53 dollars. I get roughly 60 liners out of that so they're just under $1 per cage.
Hope that helps!
jb
-----
What's written above is purely my opinion. In fact, MOST of what you read on the internet is someone's opinion. Don't take it too seriously 
Jonathan Brady
DeviantConstrictors.com
Deviant Constrictors picturetrail
