I use a couple that are made by Norwesco, not Ace, but I'm reasonably sure they are both HDPE thick walled white plastic. The place that I purchased the Norwesco conicals from also sold Ace as well.
In general, they work like a friggan' champ. You don't want to do super long term aging in them, as HDPE isn't completely vapor tight--and you have to futz with the lid to get it to seal correctly. But if you are planning on aging for two or three months in on of those, then transferring to kegs/bottles, they are *perfect*.
As for used ones? No way. Since they aren't completely vapor tight like stainless steel, they do retain some smell. If you are just putting beer in, it's not a problem. But I consider them single-job fermenters (aka the beer conicals hold beer, the wine conicals hold wine, etc.)
Oh, and the stands suck. Not worth the money, build your own out of plywood.