I have had the good fortune to have dived with a number of Carbon and composite fins over the last few years. Its kind of been a personal quest to find and experiment with new fins and theories on fins.
Before I go any further I have to say lets only talk about 100% Carbon fins. There are a number of fin manufacturers out there that claim 'Carbon Fins' but in actual fact they are a mix with glass. The reality is that as little as 20% glass will render the fin only as good as a glass fin, just a bit lighter. In most cases the 2 outside layers of cloth are Carbon and the rest glass. My take is dont waste your money just buy glass composites they work just as well for half the price!!
So when you see 'Carbon' fins for cheap ...first ask why are they cheap.
Back to the discussion now that I have that monkey off my back
The more I dive with different fins and in different situations the more I realise that there is no one fin for everything or all diving.
I have found when diving in winter with a 5mm suit and a few kilo's extra weight a stiffer pair of fins works well.
When diving in a 3mm and less weight my ultra light fins are amazing.
The problem for me is when I have to swim the extra weight off the bottom I need extra power, but as Jeremy pointed out you need more gas and that effects dive time and stamina.
I have 2 fins that I have settled with for now after testing and using maybe 10-15 pairs of different fins from different manufacturers local and abroad. (often both in pool and on a few sessions diving)
They are a medium Rob Allen Carbon fin (they are about 4 or 5 years old and in std RA footpockets)
And a pair of medium light Subcraft blades in Pathos pockets.
I wont go into the specifics of why I chose these fins now and the different manufacturing processes used in each.
The most important thing to as your self is what are you going to be using the fins for, like Jeremy said. But even then there are other factors you might want to consider aswell.
I have found that the cape guys tend to like stiffer fins, which is probably a combination of the heavier weight belts, sometimes shallower diving and having to pull Yellowtail off the bottom.
I have also noticed that some guys initially leaned towards stiffer fins because of the style of kick. I think South Africans have a unique situation in that many of us shore dive through surf and have grown up with std plastic Genesis / Picasso type fins. This combination causes you to bicycle kick, where you bend your knees to bring the blade forward and through the water.
If you try and use this kick with soft fins they just fold underneath you and are useless. The correct way of finning is with a very little bend of the knee. This is way more efficient and taxing on your legs. But try this with stiff fins and your legs want to fall off.
The guys in the Med have perfected this and dive very deep even in thick suits with very light fins. Just go watch some of the better Med divers Youtube clips and then watch some of our local vids ...... we are a bunch of rugby players.
I have had loads of guys chuck out soft blades because they felt like they did not work or were too soft. The question I want to ask is how much time did they spend focusing on their fin technique?
It took me months to break the habit of the ol' bicycle rugby player in me. And even now I have to concentrate when coming off the bottom to fin properly. That said when I go back to my stiffer blades I often don't last long before going back to my old habits. I guess you will adapt to the fins you have.
There is more to say But I will leave it there.