I have done a little casting with Smooth-On resins and silicone moulds, making copies of 28mm sculpts I made with Super Sculpey. I am hardly an expert but here are my thoughts based on that experience:
1. You can get some very nicely detailed results.
2. The resin (Smooth-Cast 220, I believe is the one I've used) seems nice and durable, can be carved and sanded, and seems to take paint well. I haven't tried glueing it.
3. Depending on the shape of the sculpt, you may have issues with some air bubbles marring bits of the casts. They tend to appear on undercuts (chins, elbows, etc.). This definitely varies depending on the sculpt; pieces designed with fewer undercuts are much less prone to it. Making your mould right will also make a big difference, as it's important to provide enough air channels (not sure what the technical term is-- small channels running from undercuts back to the top of the mould, to allow resin to flow and air to escape). I have just purchased a pressure chamber to help address this problem, but as it hasn't arrived yet it will be a while yet before I can say anything about how that works.
4. You should be able to make moulds of durable sculpts without damaging them, but if they are made of something brittle (like Sculpey, or probably FIMO) they are likely to get broken as part of the de-moulding process. This needn't be a big deal if you can get what copies you need from the mould, but if the original is important to you that may be a problem. On the other hand, a polymer clay original is probably going to get destroyed by most other mould making processes too.
5. There's likely to be a lot of flash on your casts so they'll need some cleaning up
about like what you get on a sprue of Red Box miniatures.
6. You can get a whole lot of small scale minis out of a bottle of resin. I'd guess 100's from the intro kit size bottle, so long as you're casting multiples at once and you mix carefully and don't waste much.
7. No idea how long the moulds will last. So far I've done no more than four copies of any mini; but after four the moulds seem good as new.
Hope that's helpful, and good luck!