I've been a powerlifter for years and have always done so in my converse shoes. About 6 months ago I switched to Olympic lifting and have been 'uhming' and 'ahing' about lifting shoes ever since as I was unsure as to how much difference they would make.
They may a significant difference! As soon as you put them on they feel solid. Achieving a deep receiving position is both easier and more stable. When you squat in these, it really does feel as though your feet and bolted to the ground - no lateral wobble or heel compression on the way down, just solid.
Hands down the best addition to lifting kit I have ever purchased!
With regard to sizing, I am normally a size 10 (UK), I bought and 10 and they fit perfectly. A friend of mine has slightly wider feet than mine and has reported that while initially tight (width-wise), that have since stretched just enough to be comfortable.