I haven't used jb weld, but have used epoxy based adhesives before for filling small holes where didn't want to weld, and used bodyfiller over that. You have to watch though, many recommend epoxy priming first before using any bodyfiller over adhesive, but believe that is mostly urethane based stuff.
I would not consider using jb weld as a body filler, although something very similar is often done on urethane bumper repairs, using a adhesive that has some flexability for the majority of the repair, and regular filler for final finishing work.
If I was stuck on using epoxy as a filler, well then I'd probably use an epoxy fairing compound, like I've used in the when working for some boat builders (basically bodyfiller in marine terms). Its a 2 part filler as a base and convertor must be mixed together.
Haven't really seen any bodyshops using it at all. although I am sure somewhere some place might, but fairly commonly used in the marine industry.http://shop.sailnet.com/product_info.php/products_id/38
I like fiberglass filler when a bit more fill or a bit more water tight filler is wanted then comfortable using plastic filler, but would actually prefer using the epoxy filler over all metal junk that I was forced to use a whole lot of at a past job.