From the start for me, durability was key because kiddo was getting my customs to play with.
At first I stuck with what I could find locally, so I ended up using PC Marine Epoxy Putty from my local True Value. It was about $5 for a 2 oz roll in a tube and lasted me a long time. In comparing it with other similar stuff it had the longest working time (about 15 minutes give or take). The 2 part compound is rolled together, so you just have to slice it off the roll and knead it to activate it..so there's no guess work on the amounts to combine. It was quite sticky, but I found a little cornstarch on the figures helped until it was less sticky to handle. It does have a smell, but it wasn't overwhelming, and was ok to work with for sculpting. Once it started curing it was easier to work in finer details and it smoothed with water, tho not perfectly, and you could take a dremel to it really well. REALLY durable stuff!
After seeing Vengence's work on JBL, I decided to finally try Apoxie. I got it from
http://www.sculptingstudio.com. I think there was a special at the time, but I got a total of 4 oz of Apoxie and a bottle of solution for smoothing and cleanup shipped to me for around $12. Even with all the goof ups and experimenting I've done so far, it'll last a LONG time. The compounds come in 2 separate containers so you have to approximate equal amounts, but that's not a huge hassle. I'm really only beginning to get the hang of using it (and would love any tips!), but the working time is outrageous. I've been able to work on something for up to an hour or more and then I can come back a little later and tweak the details if I want. I've found I can get much better lines and details with this stuff, and it seems to blend seamlessly with the plastic, which the PC Marine didn't quite do because it has larger particles. It also doesn't have a bad smell (if you don't mind the scent of corn chips). Dremels and smooths really nicely. Seems like it'll be as durable, or more, than the PC Marine.