I've kept mice in Crittertrail cages - usually multiple cages connected, but sometimes when I was overloaded (there was a period when I had several dozen mice as a result of a few pregnant foster females), just one cage. I would say the CT1 is sufficient for one or two mice, as in I wouldn't call it cruel or negligent. Mice are the only animals I'd ever consider keeping in CT cages, since they can utilize room in those sorts of cages that hamsters or gerbils, for instance, can't.
Still, the other posters have made valid points. You could get a larger cage for less money, and bigger is always better when it comes to cages. Plastic cages with little parts tend to fall apart and often make it easier for mice to escape. The ventilation isn't as bad as some other modular plastic cages, but it's not as good as it could be.
From my experience the best cages for mice are either aquariums or bins with mesh tops. Virtually escape-proof and the ventilation is much better, and cleaning takes a couple minutes as opposed to over half an hour, which is what it took me to thoroughly clean a CT cage.
So, if you want to spend the money (maybe to buy two of the cages), are willing to clean more frequently and spend more time doing it, and are prepared to duct tape them when the inevitably break...well, there you go.
