I've been making fairly extensive use of Instant Mold. I've not tried it with resin, for the reasons people have already stated to do with the heat. I got it specifically to use with green stuff and Magic Sculp. I've found both to work very well with it, although the Magic Sculp is less clean and leaves fragments that you really need to scrape or wash off before you re-make a mould (as mentioned already).
I've pretty much always used the Instant Mold in a press made of lego, so I could apply real force to ensure things filled in properly. When making 2-part moulds, I found the best was was to make the first half, make the second half on top, and then turn it over and remake the first half. The reason being that when you make the original first half, you can't apply proper pressure because half the model/component is sticking out of the mould. Once it's soundly encased in the second half however, you can lean on it properly when you're remaking the first. If that makes sense...
Biggest trick when using the 2-part moulds is getting the amount of putty right. Bit of an artform, really. Takes practice.