I would definitely recommend the Wacom tablets as the best supported tablets on the market. I have owned other brands, but as the biggest player in this little niche Wacom is the only company I've found that has consistent software support in all your applications. Also, its *infinitely* easier to get replacement pens, nibs, and accesories for a Wacom than just about anything else (expensive, though, so try not to lose any of the spare nibs your tablet will come with).
If you're buying second-hand, steer clear of any tablet that uses ADB connections. This is a connection format that died out nearly a decade ago, and is not supported by Mac OS X at all. The current generation of Wacom tablets use USB2 connection, which is very fast and reliable. Earlier models (such as Intuos 2) use USB1 which is also pretty good.