I own a Wacom Intuos2 12x9 (original purpose was for a DIY cintiq build, but they're much harder than to make than it sounds). I personally find it's size to be a tad annoying as it requires you to move your arm a lot more than a smaller sized tablet. I've also used smaller, 6x4 tablets which I found too hard control due to their size. Previously I owned a Genius mouspen 8x6 (this is a re-branded device, I can't remember who the OEM is though). I would defiantly say buy a Wacom because they are of a much higher build quality than anything else I've seen, have terrific driver support and don't have batteries in the pen. I would defiantly recommend say get an Intuos, however, if you want to save a bit of money, there is nothing wrong with buying an older model. The 1024 pressure levels is more than enough until your brush gets bigger than 1024 pixels. So in all, I recommend a 8x6 or medium Wacom, and remember that there is no shame in buying an old model! The Intuos line has hardly changed, even the Intuos2 has double the pressure levels of the Bambo, and there is a large age gap there!
I hope I've helped