For Painter/Painter-derived softwares (e.g. Pixia, Artweaver), the pressure sensitivity comes in really handy - everything works out nicely and the nibs really do make a difference. In contrast, using Intuos on Photoshop is not my favorite choice - the hardware's too sensitive for Photoshop and the lines get way too jittery. One could always reduce jitters by making a huge canvas and drawing on a zoomed-out surface, but my computer couldn't handle such workload.
I'd suggest using Artweaver first, and if you like digital painting, buy Corel Painter.
Oh yeah, and using a tablet helps out on 3D modeling softwares a bunch (e.g. Maya, Blender).

