I tore my left ACL in 1983, didn't know it at the time. Continued skiing the rest of that season and the next, even ran a marathon in spring 1984. But kept tearing the meniscus over that time, again without my knowing exactly what was happening. Finally locked up my knee after falling off a windsurfer in summer of 1984 and had to have micro surgery. They cleaned up the meniscus, but told me my ACL was so far gone that it was two tiny bits of scar tissue at each end. Only thing they could do (technology has advanced quite a bit since those days) was to replace with kevlar, if I wanted. I would have been laid up for a long time, followed by a long recovery. Decided it wasn't worth it.
The ortho recommended a custom knee brace, which I immediately got, Since then I've had three, current one is a DonJoy. I have used the brace every time I've skied since my surgery, and average 20+ days a year, lately 40+. I could not imagine skiing without it. It gives me stability, prevents torque and hyperextension, and most of all gives me confidence. After all, I'm missing a body part! I also use it on the rare occaisions when I play softball. I'm not into basketball or racket sports, so that's not an issue - if I was I would wear the brace, although I don't think those are appropriate activities for someone missing an ACL and loss of meniscus material.
Once the brace is on I don't even notice it. Since the days of stretch pants are long behind us, my ski pants easily cover the brace with no impediment to it's performance. I'm not a racer, but I love skiing bumps and have never finished a ski day, no matter how hard I've skied, with swelling or a sore knee. I suppose that my age will catch up with me on this and some sort of arthritis will set in, but so far so good. I could not imagine anyone taking the risk of skiing without a custom brace if their ACL was gone. Although custom braces are expensive, often portions of the cost are covered by health insurance - definitely worth looking into.