I just installed a few of these today. The design is simply brilliant. Very sensitive mechanism. Easy to insert bait from underneath, easy to dispose of mouse and clean. It should kill them pretty instantaneously without them suffering.
Update: I caught 2 so far in my garage this winter (which is a good sign, means my garage is not infested with mice! ) bought some for my girlfriends' shed and caught a few there as well. I tried the bucket trap with roller which drowns the mice. It works really well, but I don't know how long it takes for the mice to drown. I then installed 2 of these traps and it just took a few hours and it had caught another mouse. No mess, I'm sure it didn't suffer and it's easy to dispose of and just reload it for the next one. As much as I don't like killing animals, I don't like having property damage because of them either and in terms of mouse traps this is probably among the best options out there. Poison will kill the mice, but can also kill the cats that eat the mice. Sticky tapes will let the mice suffer for hours before it dies a long painful death.
I can only highly recommend these traps, they do what they are supposed to and are super easy to setup and reset once they've caught a mouse. You often don't even need to add new bait as they don't have time to eat it...