I am now just over a year post-op on an ACL replacement and meniscal tear surgery (2 for 1!) on my left knee.
My initial injury occurred when I tripped over my ego one night in class. We were doing a "show your spirit" type drill that consisted of run, jump, and yell. After doing this a few times, our instructor made a contest out of it, and on the last jump (in the finals!), I ran, jumped, yelled, hit the ground, my left knee collapsed, rolled forward, came up clutching my knee and yelled a little more.
My doctor, when I went to see him, told me I had just pulled something (yes, as a matter of fact I have switched physicians recently). After a week or so, I tentatively went back to class, working light, and used a knee brace for extra support. Around 8 months later, I was at a tournament sparring, took a step forward, had my foot catch "wrong" on the mat somehow, and the knee collapsed a second time (with the brace on). Finished the match, then "retired" for the rest of the day (fortunately it was one of the last events).
The next day, my left knee was literally 2-3 times the size of the right, and I was unable to bend it.
It turns out that the initial injury ruptured the ACL, which inherently destabilizes the knee afterwards. Even with a brace, the reinjury rate on this is something approaching 80%. The second injury tore the meniscus in the knee.
Last August 26th I had the surgery where they use a piece of your hamstring to make a new ligament, and was back in the dojo, moving very carefully and slowly, about 6 weeks later. I have gradually worked back to "normal" over the course of the last year (no sparring for 6 months at least!), until now my "bad" knee has very little effect on my performance. If overworked, it will sometimes ache a little after class, and front kicks with my left leg feel a little strange in comparison with the right, but otherwise stances, kata, sparring, etc. are all good now.
I don't recommend that anyone actively seek this out, but the recovery and stability gained from having the surgery is way way way better than living without an ACL.