Prismatics are good if you have the power to spare, and simply want the maximum shielding for a single engagement (PvP). The shield strength they offer is absurdly high, but at a price . . . regeneration time. They renegerate at snail's pace, several times slower than comparable bi-weaves, and once they are down, they'll stay down a loooong time. (Okay, you can reboot, obviously, which is a saving grace.)
I did try one on one of my combatr ships, but I do not do PvP, and for PvE, bi-weaves suit my playing style A LOT better.
That said, for certain types of ships, namely, freighters, they allow mounting fantastic defences at a much lower "slot cost," freeing up big slots for cargo racks (or whatever you fancy).