I've had 6 female mice, and I got lucky none of them were pregnant when I got them. They make wonderful pets, and I loved sitting and watching them, especially when you put something new in the cage, like a tissue (you have no idea how entertaining a tissue can be), a toilet roll or a piece of paper.
They are not as time consuming to take care of as a Guinea Pig, if you have a large enough cage, you should only need to clean it out once a week. Though when I have 4 mice in the one cage, I had to clean the corners out midweek to stop it from getting a little smelly.
I had a Critter Trail One and a glass Tank (but I hated the lack of ventilation on the tank, I got Dad to make a wooden lid with an open section that he covered with Shade Cloth (allowed more air flow, but they couldn't chew it). I much prefered the Critter Trail to the tank though, it was easier to clean, you could take it apart dunk it in hot water and dry it, replace the bedding and mice in 5-10 minutes. I also found it easier to look at (we kept it on a shelf in m book case in the Lounge Room) If you do get a Critter Trail, you'll have to buy another wheel, the one that comes with them isn't very nice, mice can go flying out of them (funny at the start, but it was quite dangerous) I ended up buying a plastic ball shaped wheel, and they loved it, plus it didn't squeek or make the wire rattle on the cage. Also, I found that the Water Bottles that came with them leaked a lot, so I bought one of those caps to over the gap, and a dripper bottle (like Guinea Pig ones, but much smaller) And you could poke it through the bars.
Bedding is alos important, when I first got them I used rice hulls, but they were very dusty and not very absorbent, and I had to clean it out every 2 days. I then switched to Aspen bedding, and it was much much nicer, it was more expensive, but laster longer so ended up being cheaper. I used the lazy lookout as a sleeping area and put shredded paper in there, they loved making tunnels and nests in the paper, and if you put a tissue in the cage, they will shred it and use it to line their sleeping areas. (SO cute to watch)
Mice are wonderful pets, and as long as you handle them often, they are fine with it. I would often sit on the couch watching tv with a mouse sitting in the palm of my hand. Except I had to hold them in a tissue, to catch any poops.