I owned the Apeks RK3 HD first as they're very popular in Kuwait and highly recommended by a lot of the instructors and divers that I dive with. However, I've tried them on 6 dives and I found them causing me severe leg cramping and foot pain and not very efficient (even though I workout, lift weights & jump rope so I'm not out of shape). So I decided to go back to flutter style fins and I picked up the Tusa Hyflex Switch Fins in Blue (small size US 7.5 UK 40) and I use them with 5mm IST Titanium boots and they're a perfect fit. They fit much better, are easier to put on and take off (I used the default medium setting on the strap) and I tried them on 3 dives where the current was extremely strong and I was finning at full power without leg exhaustion or cramping. When the current died down, I found them very relaxing and efficient with very good propulsion. Maybe I just didn't get used to the jet fin style, but I much prefer these Tusa's. Highly recommended.