Biologically slugs are merely snails which have carelessly lost their shells -- some species even have little shells still tacked on the back.
I have read that they taste unpleasant, but have not yet tested this. They do have more slime, so may froth more when cooking. Am I imagining having seen Ray Mears eating them?
However, it would be easy to try ...
If trying them, I suggest treating them like the large brown garden snails, which are even tastier than bought Roman snails (in fact both were probably introduced to Britain as food).
First check no-one has been using slug pellets nearby. Then clean the animals, by keeping them for a day or two in a container with some oats, flour or similar until their droppings are white (as in the traditional rhyme: Snail, snail, put out your horns/ I'll give you bread and barleycorns). Then boil for a short while and serve with lots of garlic butter.
Cruthinboy -- snails need lime (calcium carbonate) to make their shells. Therefore in areas with acidic soil you tend to get more slugs instead, although you may get snails near walls where they can get lime from the mortar.
I think some sea-slugs are eaten, (although sea-cucumber is sometimes incorrectly called sea-slug). Sea-slugs are molluscs too, but are not closely related to land slugs.
Please feed back results of any experiments...