I began my workout life, many years ago, with a 20 rep squat routine. My first workout I used an empty bar, added 10 lbs to the bar every session, and after two cycles (about a year) I was using 300 lbs for all 20 reps. Looking back, those reps were probably a mile high and my form was pretty ugly, but I gained a TON of size and strength from doing them along with some big eating. I eventually got 350 lbs for all 20 reps, but my depth was probably still not that good. Since then I've worked up to 300 lbs with proper depth (using a 14" box).
I am currently working on them again with a goal of hitting that PR of 350 lbs (with good depth this time) and hopefully on up to 400 lbs. That might be wishful thinking but it doesn't matter, the journey is what counts. They are fun to start with, then become really tough, and end up being a fight for life. They build real toughness and everything else in the gym just seems like child's play in comparisson.
If you've never done them, you owe it to yourself to run at least one cycle and really work hard. You will discover a lot about what you're made of inside and if you eat accordingly you'll put on a good amount of size; there is probably no better mass gaining routine out there except maybe the craziness of the 20 rep deadlift routine, which I was also stupid enough to try.