Here's an interesting site for you to read.
http://homepage.ntlworld.com/jean.wright93/mm/mm.htm
From what I understand they are rats, not mice or more properly said, more closely related to the true rats than the true mice.
We are breeding them in a limited way to feed our WC/LTC rescues as well as a few very picky breeders and I must say they have gone over very well here. Ball pythons tend to be very prey specific so we will not be feeding these to any snakes that are for sale.
As far as care they are incredibly easy to care for. Many of the benefits of mice as far as production of offspring and size but without the awful stink of mice or the problem that some mice show of cannabalizing their own offspring. They aren't however nearly as friendly or hand tameable as regular rats.
We currently have one stable colony trio (1:2) and they are clicking along wonderfully, producing offspring, eating, etc. and basically no real worries with them other than watching that the breeder male doesn't nip me. He's VERY protective of his mates and their offspring and for a little rat, has a big attitude. LOL