If it's the stuff I think it is, you have to use an awful lot of it, as it needs to form a physical barrier right round the plant or plants, which slugs/snails can't or won't cross. Fine grit or gravel would do a similar job (although they're not cheap either). Or crushed eggshells, but you need a lot of that too - each egg ony yields about a teaspoon of crushed shell.
On the plus side, the Slug Stoppa is not unattractive around plants, looking a bit like pale grit. But one packet won't go very far.
