Does it make sense to get a Wacom Bamboo Tablet for Adobe AE?

Depends on how much roto you really have to do. If you're animating a lot of shapes over a lot of frames, a tablet is a lot quicker. There are very few compositors I've met who don't mind rotoing with a mouse. I've never used a bamboo model, but they seem to have replaced the graphires, which I've used loads. Despite their small surface area, I've found them to be far more useful than a mouse (at least 20% quicker, maybe as much as twice as fast); I borrow my flatmate's whenever I work from home. Ask yourself how much time you're gonna spend masking and if its worth you investing in the time/frustration you'll save. Of course, if you're considering taking up compositing as a career, its probably a sensible investment in the longrun. i'd have bought one years ago were it not for the fact that my flatmate has one she never uses.

Besides, their resale value on ebay is really high. Higher than macs even. When I looked into getting an Intuos3 a few years ago, and found them prohibitively expensive, I found that a 5-year-old Intuos1 cost only 15-20% less online. The technology never really moves on that much, I think. There's already more dots-per-inch than anyone would ever need.