[liucipher]

It's fine to limp w/ 99. There's a chance you'll hit your set and someone will stack off to you w/ TPTK or overpair. With a pp as high as 99, there's also the added bonus that there's a good shot your overpair is best on an undercard flop.

But once someone more than minraises, I'd get out of there. It's become too expensive to play for a set. It's also difficult to assume your hand is best on an undercard flop because you could be dealing w/ TT+.