Sometimes it's tough to trap them all, and the poison can be very
effective. I have used poison with great success. Yeah, a few of the
critters will remain inside, but you'll get them all quickly. You can
trap as well, of course - any mouse the traps miss the poison will
almost certainly get. I had problems with mice in an attic, and it was
impossible to access the area they were living (I could hear them at
night, but there was no way to climb into the very small access space.
You could see it, but not get to it.)
I tossed poison in there, and after two nights of vigorous activity
(you could hear the poison blocks getting dragged around), the mice
Incidentally, the poison I was using doesn't have any "secondary
poisoning" issues - that is to say, if a dog or cat eats a mouse that's
eaten the poison, there aren't any ill effects.
If you're using the poison in an accessible location, you can purchase
bait-boxes that animals and kids can't get in to. Not that the poison
holds any attraction for animals in any case. My dog, who will eat
just about anything, shows no interest at all in the blocks.