Our cats were good hunters, but the groundhogs were smart enough to steer clear when cats were in the vicinity. It was a neverending battle just defending our garden and dikes from groundhog destruction. I probably killed 10-12 of them every year, but new ones would just keep moving in.
Another use for those things that dawned on me (I'd buy 'em regularly due to two sadly diapered parents) is to tape them to the garage floor in the parking area if you have a small pesky oil leak.....beats the hell out of putting kitty litter in a cookie sheet - which wifey would always run over.
Works like a charm. I've had one down for six months and it's good to go thru the 2010 election cycle.