I've tried a few:
WinTime Paintpal.
An older model Aiptek 12000 (Rebadged by many, Aldi, Lidl etc)
Wacom Volito A6
Wacom Graphire 4 (A5)
Intuos 3 A5
The Wacoms are the best by quite a way, so smooth and responsive. But, they are bloody expensive. It depends what you want to spend but I'd say the best value has been my Graphire XL, £25 off Ebay. It's damn smooth, not as smooth as the Intuos but not far off. And at the price of an Aldi Aiptek, I'd go for a Graphire 4.
It depends are your setup for the size, I use dual 4:3 monitors and sometimes have the tablet 1:1 on 1 screen only, or proportional over the 2 screen, 'wasting' an area of unused tablet. A wide screen ration tablet may help with my setup but to be fair I don't really notice and A5 covers 2x 21" 4:3 monitors with enough resolution.
My Aiptek has A4 working area and I just find it big. There's lots of arm movement on an A4, unless your slapping Painter art around maybe.