well, this is my personal experience, i can't speak for anyone
i finished Intro Cave, and then took DIR-F
i found DIR-F invaluable. i got to concentrate on trim and bouyancy, and work
on them, and work on them. the videotape was an amazing learning experience.
just getting videotaped was worth the $300 bucks price of admission.
why?
well, we didn't really cover fundamentals in all that depth in Intro. Cave. i could
get by (and did) with mushy skills; quite rightly so, my instructor said i had
a lot of work to do with my skills. we just didn't have time to work on basics
AND work on intro. cave over a weekend.
so... DIR-F was awesome b/c it really focused on those basic skills that make
everything else so much easier
but... if i were full cave... the way i've seen most full cave divers dive... probably
those fundamentals would have been in place by then. thus, a full cave
diver probalby wouldn't have gotten as much out of DIR-F as i did