Training quads is a bitch.....a lot of times if you are really pushing as hard as you should be, you will get a severe case of DOMS,sometimes for days after training legs as opposed to any other bodypart.
I know people will say my legs sucked which they did, but I ALWAYS trained the living fuck out of them.
I also agree with Jeff.....when it comes to squatting,your lower back is a major weak link.....I was a pretty good squatter for a light small boned guy and they personally never worked well for me regardless of weight being heavy,high reps,low reps,all different kinds of intensity techniques thrown in.....the result in my case is that now I have three bulging discs and arthritis in my lower back and also in my neck.
Sorry to ramble.....my coffee just kicked in.