I've tried Force Fins. They really aren't all that good, IMO. They do allow folks who have a bicyle kick to dive more efficiently without correcting their technique, but they would be ever more efficient if they simply fixed their technique and stayed with conventional fins.
I am glad that you put the DISCLAIMER "IMO" on that.
1) a "bicycle" kick is the kick that you can't use with ForceFins, as flexing the fin on the downstroke is how you activate them, which a bicycle does not have
2) the best kick for ForceFins is a true swimmers straight legged kick, not the bent knee kick that most conventional fins use.
After diving Forcefins for over 10 years and close to 1000 or more dives, I have found they are simply the best fin on the market and their limitations in Recreational Scuba diving are few and far inbetween. They can out run the fastest Apollo or Atomic Splits, they have better control than the Quattros, they keep working even if you break the heel strap, and you'll likely NEVER cramp with them on.
I have lent mine to over a dozen divers over the years, and without exception, if they used them for more than 4 dives, each of them purchased a pair for themselves and one guy is yet to return them some 5 years later. I have only had 1 diver use them and not buy them, and he used them for about 5 minutes and said he couldn't feel them, I tried to convince him that not "feeling" them is a good thing, but... oh well.
As far as my opinions, just swap over to the
FF thread and do some reading, I have reviewed almost every fin in the Prime Line and posted my results and comments. Check out
http://www.scubaboard.com/forums/force-fin/230415-picking-right-fin.html
http://www.scubaboard.com/forums/force-fin/221715-inside-out.html
As for efficiency, the ForceFin original and the Pro was tested back in 1993 in a US Navy Sponsored test, which was later classified because "is likely to cause substantial harm to Force Fins competitors". Some of censored results, comparing only to the closest competitors is on the FF website. Plain and simple, they were the most physiologically efficient fins they had ever tested, which is why the following people use them
John Chatterton
Jean-Michel Cousteau and his entire Ocean Adventures Dive Team
Jonathan Bird
Hal Watts
the majority of Navy Seals
Japan's, South Korea's, and many other countries divers and special ops forces.
commercial divers and hundreds (of thousands) of regular divers the world over