I use a Wacom Intuos 3 A5 Wide and it's really really good... Much more accurate and more sensitive than the cheapo iball etc. tablets that you get... If you're going for a cheap tablet, a Wacom Bamboo or an Intuos 3 would be a good idea (I'm assuming Intuos 3 prices have gone down after the release of Intuos 4)
There really is only one good choice and that is a Wacom, although a little more expensive they really last. I have had my Wacom for about 6 years and it is still working like it did when I bought it. I have personally tried other brands and they just don't hold up, the Wacom I use has over 1024 levels of pressure and I am only using the 5x6 Intuitos 2 XD. This is an older model and I am sure there have been improvements. There are other threads about this that have been posted on TE before so you could do a search and see what other people have written about it
Oh yeah... and one more thing Wacom is the only brand that has drivers for every flavor of windows and also for Mac and has an incredibly good open source driver for Linux too.... truly cross platform.