Which one? There's several running right now.

Normally, if its a normal 2-week bundle, if you like anything in the first tier, it's worth it. The BTA or top tier may be worth it if there are several games you want in the bundle, and you feel the value is worth it (also, its for charity, and you can choose where you want your money to go). As for the Monthly, I feel you should only get it if the early unlocks interest you.

Hope that answer helps you.