My friend is a world class masters swimmer from Russian with top 10 performances in  breast stroke and IM.  She uses these fins to develop more powerful kick.  She puts fins on for kicking sets, and breast stroke sets.  Other team members in Russia always want to borrow the fins from her that's why we needed all three sizes to outfit everyone on the team.
Excellent delivery service - on time, right product, no problems.

these sizes are comparable to the optimus fins sizes.  This is a great product as many of our swimmers have used them to help correct their kick and it has worked.  I would highly recommend this product.

Works well for what i need them for.  They hurt after 400 meters of kicking but that's all I really need them for.

They work my legs more than without
the strap cuts into my ankle. They need to be softer or more padded around the ankle

I bought these pair to help my son on breast stroke kicking. It was fun at first but he complaint that they really wore his leg down. I tried them and like them but they we a bit too big for us on the size M. I think that Speedo got it right on these breast stroke fins because they would force your feet to turn out at the same time force you to really squeeze your legs; otherwise, you would go nowhere. The rubber strap that goes around is a bit too hard and could really rub against your ankle which could really hurt you. My son complaint about that the fins hurt him.
Overall these are good stuff.

Keep getting crazy looks at the pool but these things really work. I have mediums for size 9/9.5 feet. Fit great, have worn them up to 1000 meters with no discomfort other than screaming leg muscles!