I couldn't control my tablet worth shit when I first got it (year and a half before osu was created maybe?) but after a few hours of it tablet skills see a formidable improvement.
I don't know whether or not I'd get a tablet
just for osu! - I originally got my tablet for drawing, osu! was an unplanned bonus. But I
vastly prefer playing osu! with a tablet instead of a mouse. Granted I'm more used to it, but if I were to paraphrase, I'd say the tablet is like a more controlled version of the mouse. If you happen to have a projector you can hook up to your machine, then I'd say by all means yes get one. Shine the projector image onto the tablet canvas and it's basically like a touchscreen - the optimal solution.