It's possible but I don't recommend it. I tried playing TF2 with my A4 tablet and it wasn't as good as using a mouse (played sniper class, for obvious reasons ).
A tablet is just an absolute-positioning mouse after all.
You can play games like COD4 and other first person shooters really well with a laptop track pad, you just need a few weeks to get accustomed to it, but a graphic tablet is taking it a bit too far