I think everyone starts off using their mouse first. I know I did too. I ended up switching to a wacom bamboo tablet because my hands kept getting sore from using it to draw. A few people here have already mentioned this too, but it also depends on what you're doing. Since I tend to do digital drawings/paintings and detailed illustrations, a tablet seemed like a better option for me.
Depends on what I'm doing. My tablet has been broken for my last several deviations (3d works) and it hasn't significantly effected their quality or process time. But (aside from vector work or pixel art) I'd never use a mouse for 2d illustration.