I bought these as I found that my much loved XL Scubapro jet fins did not fit on my new rock boots! (Scubapro HD Rockboots size ML) Yes these are pricey, but I was knocked out by the quality and attention to detail of the F1 fins. They are supremely well made and very solid ( and heavy) fins. If you like like jet fins , you will love these. Sizing is very similar to the S/pro jets. The foot pocket width and depth is exactly the same as the traditional jet fin. The difference really lies in the "height" of the foot pocket, which allows my chunkier rock boots to fit. This said, I don't think the XL size would fit anything bigger than a size 9 uk boot? The spring straps are brilliant. Very easy to get on and off. Performance in the water is fantastic. Great finesse and control. Frog kicking is perfect. Flutter kick is strong but takes a bit of effort due to the size and weight, but that's jetfins for you! I think these will last me until I hang up my fins, so the price does seems worth it! Only down side is the pretty strong rubber smell! Overall I'm well impressed with these fins.