well I definitely know what you guys mean about hurting your hand with the mouse.
Having practiced more with the Bamboo since, I'm having a huge problem just getting the tablet to draw in the direction that I think it's going, almost like using an old ball mouse. To that extent, the mouse feels more like a physical pencil to me than the stylus does.
but proper paint-like shading simply cannot be done with a mouse -
I think shading is one of the easier things to do with a mouse. http://www.newgrounds.com/art/view/acethesupervillain/reaper -shinigami <- done with mouse. That said, I don't much care for a real "paint" like result, and I don't shade with the computer tools the way I would with pencils, and now that I think of it, I hardly ever use colors on physical work.
I suggest practicing with a tablet for the same amount of years you spent on drawing with the mouse and then draw another portrait for comparison. ;)
^ That would be over a decade unfortunately.
My main reservation about using a pro graphics tablet is the cost. I could spend $1000 on much more useful things if it's only going to be slightly better than the mouse I already have.