Since this is the Force Fin forum, most people who visit here will already be converts (or undecideds). To see what’s said by some others, consider such threads as:
http://www.scubaboard.com/forums/showthread.php?t=175113
Most of the opposition takes the form of “I don’t like them”, “such-and such a stroke can’t be done in them” or “they don’t have power”. Similar comments are made about split fins.
Taking those arguments in reverse order:
Power: One test is to swim laps and time yourself. If Force Fins (or splits) don’t have power, they won’t give you better times. I think people who say they don’t have power are reacting to the fact that they don’t seem to provide much resistance to kicking. (I consider that a good, not bad, thing). Granted, the test is not completely scientific because changing strokes prevents it from being a double blind test. My results are that the only pair that beats my Force Fin Pros is my Atomic splits, and then just barely.
Strokes: Specific strokes aren’t as important as what you want to accomplish. Granted, I can’t back up as effectively with anything but my paddles. Otherwise, I can do everything I want to do quite well, and usually best, with my Force Fins. These include acceleration/speed, tight turns, helicopter turns, not disturbing silt, and surface swimming. The techniques are a bit different, but can still be very effective.
Dislike: I can’t refute that.
I briefly mentioned splits above. Although I get a bit more speed from them, I haven’t mastered many alternate strokes with them. And they take more room when packing for a trip.
Why don’t I use Force Fins exclusively? The LDS where I teach doesn’t carry them.
So I use inexpensive full-foot paddles in the pool. I use splits or open heel paddles in open water with students. Which paddles? Aqualung Blades that I converted to years ago from my rockets and jets. For all other diving, it’s my Force Fins.
Yes, I’m a convert. Besides what I said above, they’re the most comfortable to wear and are extremely durable.