I have a Wacomb Graphire 2, 6" x 4", and it suits my needs rather well, considering I draw small. It was only about $100, which is a decent price, and it works wonderfully. If you draw bigger, though, they have larger sizes, but this is nice for me because it's so easily portable.
Resident Artist at: http://www.suikosource.com/
Second to Nun: A Chrono Crusade fansite