I had mice last year and i was told by the pest control guy they don't work (after buying one!). There was nothing in it for him to say this, as he was getting paid anyway.
He put bait in a container in my bathroom to see how they were getting in, and they were getting through the tiniest hole you've ever seen - probably no bigger than a pen width. They are very clever apparently and have become used to how humans live, feeding at night on any food they can find. They are particularly partial to cereals etc and will easily find a way into a cupboard even if it is above a worktop.
After finding where they were getting in (i spent countless nights afraid to go into my bathroom unless necessary), they removed my bath and blocked up all the holes. But i now hear other residents are having problems, probably because they can't get into my home. Its a long process, but i'd definitely recommend getting the bait in containers as they feed on it and take it back to the nest.
Good luck
