The best protection, of course, is to not get hit.
Gary shoots professionally, for magazines, and of big-money tournaments. The tourneys are incredibly fast-paced and paint heavy. When there's a $50,000 prize on the line, the players shoot first, the shoot some more, then shoot again, then pod up and shoot some more, then call a ref to check, then ask questions later.
In the typical airball tourney field, too, the photographer is limited in movement- the netting keeps him from moving too far back, and the boundary lines keep him from moving onto the field.
So with those three things- needing the top shot for the mag, the sheer volume of paint, and the limited range of motion- guys like Gary take some nasty hits.
Now, if you're doing it for fun, as I do, you have to think of the camera first. That paint coming right at you might be cool to see in the photo, but for us average guys, it ain't worth the loss of an expensive lens. Duck, get out of the way, turn your back, whatever you need to do.
Woodsball and recreational are usually easier still- there's less paint flying, the photographer has more freedom of movement, less tourney/money mindset so there's less "shoot first, apologize later". I've taken several hits on an airball field, I've never taken a single one in the woods.
I have a clear filter like Gary- 'cept I paid like $17 for a cheapy, since it's just ablative armor to me. A more expensive filter may be somewhat optically better, but personally, I can't tell the difference.
I did also make some armor for my camera, out of an old mouse pad and a strap of aluminum. It fits nicely, but I haven't used it yet. Leaves plenty of room for using the controls, but doesn't cover more than the rearmost inch or two of the lens. I'll post a photo one of these days, but I'm in the midst of some computer problems.
Basically, your best bet is to just stay out of the way- and wear blaze orange.