I squat 100kg, I could go more, but on a smith machine. I do 4x8 and after that some leg press and extensions for hamstrings and I can also barely walk the next day. I think that's normal for leg work as it's the biggest muscle group and the legs get strongest first. I have a feeling I can add 10kg every time I do squats.
Though I've started to ease up on leg work since all my jeans are becoming real tight.
Knee pain was the reason I got into the gym. Strengthening my quads helped somewhat I don't have the same pain anymore but walking up and down the stairs is always difficult. I have the feeling my knee will detach and my lower leg will be left hanging.
If you have knee problems, I'd defo recommend doing squats on a smith machine as it isolates the quads and relieves the knees. The purists will call you a wuss, but it's got it's benefits.
Funny story, how I injured my knee. Here's a hint:
http://www.youtube.com/watch?v=UhUrOC_NHZM