I've noticed quite a few slugs around my garden recently, and I realized that I don't know if that's a good thing or not. I know some bugs are bad and some are good, and I don't know what category slugs fall into.
Slugs are one the most common garden pests. If not eliminated they can do much damage. They do most of their damage at night when you are not watching. If you see one then surely there are many that you don't see. There are many products on the market for slug control. These are mostly baits. The easiest ways are available around the house. The first is beer. Slugs love the yeast in beer. Bury a container in the ground where slugs can climb in. Fill it halfway with beer and the slugs will climb in and the alcohol will kill them. Ashes and salt also can be effective. Ashes from the fireplace can be sprinkled around the garden perimeter. Same can be done with salt. These practices discourage slugs and snails.
Slugs are considered a pest. I don't know of any benefits they provide.
See answers to this question for ways to deal with them.
I know that you have already accepted an answer, but I thought some balance is needed, given that the other answers are all along the lines of "KILL THEM, KILL THEM ALL!".
Most slug species are not problematic and play an important role in breaking down material in compost heaps. Some species even eat other slugs. They are also an important food-source for a range of birds, many of which will help control populations of other pests.
There are some species that are particularly problematic pests though, and you will want to control these. I would avoid using any of the pellets available, even the so called organic ones can be harmful to other wildlife, especially cats and hedgehogs. Salt can be problematic as it is harmful to plants. The most effective control I have found, have been beer traps: they effect only actively foraging slugs (i.e. the ones that are mostly likely to do damage) and they are reasonably specific in the area they effect.