Lots of good advice so far.Try them, see if you like them.
The black twin jets are, I believe the stiffest and heaviest of the Twinjets.
The grays (at least the original grays) are much less stiff.
I have used the black twin jets for 5+ years, and I really like them. Some observations/suggestions,
opinions (FYI:I'm a recreational diver):
On those "4 dive, dive days" I've had much less knee fatigue /soreness (on my old, injured,arthritic knees) than with My Aqualung/USD "Blades (which I also liked a lot).
For kicking: a smaller, slightly faster flutter kick works nicely. Be aware that it may
feel as if it takes a couple of kick cycles "to get going".
Also a frog-type kick ( I have not had formal training in the exact frog kick) can work with these fins, due to their relative stiffness.
They have worked for me, in currents described by divers familiar with the site, as strong. (They did not mention any reference speeds.)
As I said, try them, and as others have said, they were a gift.
Good luck,
Mike