It seems to me like if you're the type of person who is invested enough in the Prince of Persia series to long for The Forgotten Sands, you would likely own Warrior Within already. If not, Steam wants to have a word with you real quick -- pre-purchasing this latest title gets you 2004's installment for free.
Personally I dug the game, even though the seemingly random shift in tone was not entirely appreciated. The chase scenes -- and the basic idea of an unstoppable entity in general -- was cool to me. Still, I assume I'm in the vast minority on this one.
This offer applies to both the basic and costlier version of Prince of Persia: The Forgotten Sands, and in case you wanted to know but were too lazy to look it up, Warrior Within is a $9.99 value on Steam. About right for a reservation bonus, I'd wager.