I just did my first split-fin dive at Epcot.
They performed as advertized - when I kicked my feet, I was propelled forward. Oddly enough they did not feel soft or squishy. In truth, they felt sort of like they were simply not there, although I moved through the water with greater speed than I would have if I had not had fins on. Please don't misconstrue this as a favourable review - in my uneducated opinion the same effect could have been achieved had I sawed off half of the blade on each of my regular Mares Avanti Quattros, thereby lessening the purchase of each motion and decreasing (apparently) effort but to lesser gain. Less effort, less propulsion. I can achieve the same saving in effort by kicking slowly. In split fins I found helicopter turns to be different, but doable. I personally found a backward kick was impossible to learn in the 40 minutes allotted to the dive, and the absolute lack of current made this more pronounced.
Shorter answer: I didn't hate them - they felt sort of "not there" (especially when doing a back-kick). I will also not be buying a pair as I personally don't see the advantage. I would recommend them to a snorkeler without hesitation.
Longer answer: My analogy is that, let's assume that you have a 12-speed bike. Kicking without fins is gear 1. Kicking with split fins is gear 2. Kicking with "normal" fins is speeds 3, 4, or 5. You still have speeds 6-12 untouched (unless you are one of those free-divers with crazy fins). Any of speeds 1-6 might get you up a steep hill. It is up to you: do you want to pedal faster but easier, or harder but will a decreased cadence? What is better for "handling?" What is your optimal "setting" for an "average" dive? I opt for a higher gear ratio and lower cadence, but you may not.
Dive what you want, folks. We all own bikes with different gear ratios. Just PLEASE stop with the pseudo-science.
Real science is A-OK.