I wanted to build a biped using cheap servos from TowerPro. Before I went ahead and buy 20 servos, I searched the forums to see if anybody else succeded using them for a biped, but I found they are of a less quality and they vibrate a lot around the hold position. After I got a couple servos to do some tests, I found that the electronics are the weak point. I replaced the electronics with ones found in GWS servos and that greatly improved the servo performance.
Conclusion: for a hexapod, biped or any robot like that, high quality servos are desirable, or your robot will perform poorly. You don't have to buy the most expensive servos, but get at least GWS, where Futaba and Hitech are the top quality. I think there are other brands for digital servos that might work, but I didn't get info on them yet.