We're required to wear it at work on the range. We all wear the Second Chance [url=http://www.secondchance.com/prod.asp?action=prod&prodID=20]External Assualt Carrier[/url], which works quite well. It has large external pockets, an ID panel on the back, and is fairly comfortable. When I'm teaching at academies or in-service at other locations, I always wear it.
I've had one student turn around and point his weapon at me ("my gun isn't working..."), and I prompted "convinced" him that that wasn't a good idea (drew down on him and said "mine is!"). While I'm less concerned about being hit by a direct fired round, I am more concerned about being hit by a round that has hit something, like the metal plating, target holders, etc, and bounced back. I have a friend who is also another LEO, and he has a round still stuck in his arm after it went completely down range, hit an upright for the baffles, bounced back, and hit him in the arm - going all the way to the bone.
When people ask us why we wear armor, we tell them this: why does the xray technician wear a lead vest? Because they are around it all the time, and want to minimize their exposure. Same with us. Since we are on the range more, the chances of us getting hit by something is higher.