It took me so long to come around to making this switch simply, because I'm stubborn and resistant to change. I've always squatted high bar, and this, plus the fact that I was competing pretty regularly, made me unwilling to entirely change my squatting style.
HOWEVER, I just started reading Mark Rippetoe's Starting Strength, which advocates heavily for low bar squatting, and I don't have a competition on the books any time soon, so I decided to give low bar a real try. (I've tried once or twice before very halfheartedly, and I gave up very quickly when it didn't feel the same as high bar... but of course it wouldn't feel the same. It's completely different.)
I am extremely glad I decided to try for real, because I absolutely love how they feel, and I now plan on switching my competition squat to low bar. Why? Well, in short, my tries with low bar have all felt more like a squat than anything I ever did with high bar. They just feel right, and there's no pain (knee or low back) at all associated with the movement for me when using low bar, and I can feel the work coming from all the right places.
Notable differences for me:
1. Much less stress on the low back: high bar squats, because they demand a more upright torso/puffed up chest, seem to actually facilitate lumbar overextension. (Stick out your chest and feel what happens at your low back without intentional control.) With low bar I find it much easier to control for overextension while in the top position, and there just naturally seems to be less of it during the descent and at the bottom position. This is an extremely big plus for me; someone with lumbar lordosis.
2. Much less knee cave, and much easier to control for knee cave: in my high bar squat I was noticing that my stance became less and less consistent and less and less easy to control. Like, I would set up in a stance that felt comfortable, then I'd look down, and one foot would be turned out to ~35 degrees, and the other foot would have basically no turn out, and try as I might, I just could not get my feet even much less to STAY that way. This imbalanced and uncooperative stance made knee valgus nearly impossible to avoid as the weight got heavy. My feet didn't know where to be, so as a direct result, my knees didn't know where to be either. However, because of the downward eye gaze with low bar, I find it much easier to get my feet where I want them and to keep them there. (I'm replicating the squat form taught in Starting Strength, which suggests feet at shoulder width and turned out at 30 degrees.) This enhanced control over my stance, and turnout of the stance itself, has made it invaluably easy to avoid knee cave.
3. Hips UP vs. forward: because the low bar squat demands a more horizontal back angle, the hip drive naturally comes from a motion of pushing the hips UPWARD, whereas in a high bar squat with a more upright torso, hip drive more naturally comes from a forward motion. (Driving the hips UP in a high bar squat would cause the torso to tilt forward and the bar to lean forward of the mid-foot throwing the whole movement off balance.) Hips UP hip drive has proven to be and feel much more powerful in my personal experience and according to Mark Rippetoe's Starting Strength.
All in all, I'm very excited about this transition!