You can use the search button and find many threads on split fins.
Rodales
www.rodales.com has done some extensive testing of the fins and has found that split fins are more efficient that standard fins -- divers can achieve and maintain higher top speeds, and they also use less air using split fins.
They are much easier on the legs and knees than normal fins. When first trying out split fins, I had to keep looking at the bottom to see how fast I was going because it just didn't feel like I was putting any power into the fins, but in reality I was zooming along at high speed.
They do have some disadvantages, though. They tend to shed vortices that travel quite a ways and tend to stir up silt from the bottom more than regular fins, particularly when used by a diver that is overweighted and finning upward to compensate.
They are not as good for alternative fin strokes, such as frog kick and backwards finning.
Also, while quite efficient, when start finning from a standstill it takes a few kicks to get up to speed.
They work best with a smaller, faster flutter kick than a normal fin, so they make take some getting used to if you have used big blade type fins in the past.
You definitely have to try them out for yourself. Lots of people love them. I found them great for long swims, but in most of my diving I do a lot of maneuvering and ended up staying with standard paddle fins.