I use a Wacom Intuos 3 6x8 and I simply can’t believe the difference between this and a mouse. I tried the bamboo, and it is a toy compared to this. The differences really become apparent when using Photoshop and, as I am now finding out, in ZBrush. This tablet also comes with a wheel mouse that you can use in programs like 3DS Max (which does’nt really work well with a tablet)
If you are wondering which size to use, then I suggest calling Wacom customer support. They helped me alot in my decision.