Please critique my squat form

Here's 2 videos of me doing 160 x 5. The second set I'm a little more fatigued.

First, do you think I'm going deep enough?

Second, I noticed that my back is rounding a little bit at the bottom portion of the squat. Is that a sign of going too low?

Third, am I sitting back far enough as opposed to sitting down and engaging my hams and glutes?

Fourth, my knees are always kind of achey right below my kneecaps when I'm done lifting for the day. I mean, it's not horrible pain, but they just feel "beat up". Is that normal for squatting. I bought knee sleeves a while back, but I've decided not to use them.

I appreciate your time and all your help. Thanks a lot.

http://youtube.com/watch?v=0B6NrXGAMCM - set 1

http://youtube.com/watch?v=EKa2yRTFUhA - set 2

You're plenty deep, but your low back is relaxing to get you there, your knees are dropping a little forward at the bottom which indicates that your hamstrings are relaxing just when you need them tight. You're not standing up between reps and you're stopping at the bottom instead of using your rebound, since you really don't have any because you relaxed your hamstrings, and you're wiggling around way too much at the top between reps too. But the main problem I see is that you're lifting your chest instead of driving your hips up, thus making it very hard to generate any power out of the bottom.

I see. Of all the things I read in starting strength I still don't know how to engage my hamstrings. I just can't seem to get the feeling of it. Would maybe pushing my butt back further be the solution? It's one thing to read about it, but it's another to actually feel it under the bar.

Also, you think my chest is driving up faster then my hips? I definately didn't realize that.

Do you recommend sticking with this weight until I can get perfect form with it? Or should I drop back down to say 135 or something?

Thank you very much for your critique.

The best way to learn to engage hamstrings and all the other posterior chain muscles is to get in your bottom position and have a coach put his hand (her's would be more fun) on your low back, and then drive that hand straight up in the air. Drive the hips up, not the chest. If the chest goes up and changes the back angle, the knees have gone forward too. If this happens, the hamstrings have shortened from the knee-end, taking away from their ability to pull the hips into extension, which is the essence of hip drive.

Thanks a lot. I will focus more on my hips moving straight up, and I'll post another video later on. I appreciate the advice.