It's actually a little bit ridiculous to be perfectly honest. BioShock, BioShock 2, BioShock Infinite, The Darkness II, Mafia II, XCOM: Enemy Unknown, The Bureau: XCOM Classified, and Spec Ops: The Line -- all for just $20. Jaysus...
As always with the Humble Bundle there's multiple different ways things can be done.
You can pay what you want, but if you pay less than $7.05 (the average price paid at time of writing) you'll just get BioShock, The Bureau: XCOM Classified, and The Darkness II. Pay more and they'll add BioShock 2, Spec Ops: The Line and Mafia II to the package. Pay over $20 and you'll also receive BioShock Infinite and XCOM: Enemy Unknown.
Paying $20 for all of these games is a pretty incredible deal by any standard. That's what I'd be going for, particularly if I hadn't played most of these games already.
A percentage of the money paid to The Humble Bundle goes to charity, the charities in this case being the American Red Cross and Action Against Hunger -- two very worthwhile causes. This whole thing seems like a win-win to me. Get to it!