I have used the Poseidon Trident Fins for a few days now and I have to report that they are extremely comfortable and just as powerful as the RK3's that I had been using if not a tad bit more so.
The edge of the fin with it's softer rubber makes it a comfortable, snug fit on the foot. It is as though the fin molds to your foot. No matter what style of kicking I was using, the Tridents Fins were accommodating and managed the job without any "cramp" effect from the fins.
All in all I have to say these are my new fin of choice when diving.