They're not particularly messy. If you make sure that you clean out old food daily, and remove droppings as and when you see them, they remain fairly clean. They get themselves a bit dirty by burrowing, but I'm convinced that mine do this just to get a bath! They can smell slightly, but only in an earthy, vegetably way!
How Do you post snails? Won't they be hurt?
Most people post snails with no problem by sending them in an old ice cream or margarine tub. First, you fill the tub half way with soil, place your snails in, and fill it to the top with soil, so that the snails are nestled in the middle and well-protected. Secure the lid and bubble-wrap them and they're fine in the overnight post!
How do I stop my snails from breeding?
You can't. But you can stop the eggs from hatching. Once your snails are of breeding age (from about 6 months and older) you can regularly check the tank for eggs (which may be buried). Remove the eggs and freeze them overnight in a sandwich bag and then just throw them away. Simple! If you miss any eggs, and they hatch, there are always people who will take them off your hands for you.
What if I'm going on holiday?
This is a remarkably easy problem. If you can take them with you, take them! Get a portable tank (or use your plastic critter-keeper thing) and just take them! They travel remarkably well. If you can't take them with you, find a friend or neighbour who can snail-sit, and explain to them the simple guidelines for feeding. Make sure you clean them out before you leave and your snail-sitter won't have to touch the snails or clean them out.