as an aside...
there are a number of top skiers on the US Freestyle Team who use Don-Joy braces as a preventive measure, and Jonny Mosely is one that comes to mind. How often he uses them is yet another issue, but what I read about them is that many of the Team members use them in training.
My own experience is that the better designed models (I've used Lenox Hill, Townsend and BREG) do help stabilize the knee. Most especially, they have a physical stop that mechanically prevents excessive tibial draw and hyperextension -- two things that well-trained and strengthened hamstrings can guard against, but cannot prevent.
The experiences of ACL recon patients I know or know of have varied widely. Some never used a brace, some use them in bad conditions or in early season, and some use them always.
JohnH, who used to be a frequent poster in here but is sidelined this year with a new baby in his family, had ACL recon recently and skipped the brace entirely. He reported excellent stability last season.
I use one on each knee, but I have skied without them. Call me a 'fraidy-cat, but I've been through 2 ACL recons and I don't want to go through a 3d one. Besides, they don't seem to hinder my performance any.