I don't see any reason they wouldn't work. Technically anything that can safely hold liquid can be used as a fermenter. I like PET carboys personally because they are safer than glass, less permeable than buckets, and because they're transparent I can see when they're clean and can easily guide my racking cane to exactly where I want it. Also I rack using orange carboy caps, which wouldn't work on a bucket or one of these jerry can type water containers.
The last 3 PET carboys I bought a few weeks ago were $16 each.