I've only used it to cut monofilament a couple of times so far, but one time it was rather exciting.
I was swimming along ogling my new computer when something tugged at my fin. I turned around and saw a good sized piece of mono fishnet hung between my fin buckle and a rock. As I reached back to clear it, I lifted a stone fish off of the bottom that was caught in the net between my fin and the rock. He didn't like that at all, trashing about, bumping against my fin foot pocket and generally scaring the $#!t out of me. I pulled my hand back, grabbed my knife and cut the net loose from my buckle, all the time hoping the net didn't come loose from the rock and let him reach my ankle. Fin cleared, so far, so good.
So then, after I calmed down, I undertook a mercy mission to free the stone fish. I found a large rock and laid it on the net and pushed toward the fish with my knife. It pinned the fish to the bottom, so I could try to cut him free. Every time I would nearly get him free, he would begin flopping around and get entangled again. After 3 or 4 tries, I gave up and freed the end from the rock, pulled the net from under rock I put on it and let him swim away trailing the net.
Unfortunately, he turned back and stung me in the arm, boy did that HURT... I swam quickly, but deliberately toward the beach, but to no avail... the pain got worse and worse until I died halfway to the beach...
(it's better in person with liberal amounts of beer applied)
... actually, the last I saw of the stone fish, he was swimming south into deeper water... but the alternate ending always gets me a free beer!
p.s. I did go home after the dive and review the effects and treatment for a stone fish sting. Now, I always take at least one cutting tool on every dive, no matter how simple the dive seems.