I agree with Pull on pretty much all of that.
I have twinged, or downright hurt my back more often with a belt on than without. This isn't just because I'm using more weight; it's because I'm more careless about racking and unracking when I'm belted, and more cautious when I'm beltless.
Having said that, personally I like a belt on for anything over 350 lbs squats, 400 lb deads. I use a relatively tight belt for squats and a much looser one for deads. I get a lot out of using one - I think probably 40 pounds on squats.
I will sometimes substitute in blocks of beltless squats as a distinct variant. It really feels like a different exercise for me.
Lifetime best: 500/363/573 @ 220 belt only
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