I wear body armor all the time riding both my Class I and Class III bikes in the Hollywood Hills, whether on trail or off, whether it's a little three or four mile fitness ride or a 32+ mile trek out to the Verdugos. I'm on blood thinners, so my main concern is internal bleeding, though road rash could be a problem too. It's weird, I just don't bleed that badly, even when my INR is in the 'target' range-- dog bites, T-boned in car wrecks, and big drops body boarding. But anything that can decrease the risk of bruising, internal or external bleeding is a good thing for me.
Living in Los Angeles, we have a lot more variation in climate, so I realized I really needed two different solutions, one for warm weather, one for cold-- it does get shockingly cold in the Verdugos, can go from the low 70s to the 40s within about two hours even in the spring or fall.
Cost WAS definitely a factor for me. In fact, I got the cheapest gear I could find! The first option, the lightweight armor with mesh, can work over a T-shirt. It looks like a total piece of junk, I doubted it would last a season, but I've been riding with it four over two years. All the zippers work, no tearing anywhere. I would recommend a more high-quality version of this -- like Indianjo says, it's motorcycle gear, motocross specifically.
The jacket with armored inserts is for colder weather, probably not for you, but it has been flat-out fantastic, best $40 I ever spent, still a bargain at $60. Over two years, and not a loose seam or cross-threaded zipper-- and this, regrettably, did get tested. Was riding angry one night, went up a trail that was too steep and covered with leaves, lost traction completely, slid backwards, fell on my back and the bike fell on top of me. Zero injuries from the ground or the back. I did throw out my neck and needed PT for that, but my neck wasn't covered by my jacket. Took about a year for that to resolve. I think the armored padding would slow down a knife, and provide at least some feeble defense against mountain lions, though that's not what it's meant for, and those are low-percentage threats even when I'm riding in the inner city, by the LA river, in abandoned industrial areas or near homeless encampments. Likely better than denim, and way better than nothing.
I will say this-- both types of armor do get a lot of strange looks on the road. Expect to attract attention. You look like a character in a science fiction movie, which sometimes amuses people-- or frightens them, particularly children. A friendly wave or laugh in return usually solves the problem!
I have thought a lot about knee and leg protection. I think I only need it for the more serious trails in the mountains, where I do sometimes see mtb or eMTB riders wearing shin and knee guards. One guy I met keeps them in pack for the ascent and wears them for the descent. If you find anything interesting and lightweight, please post back.
As a practical matter, I never actually wear my respirator with armor on the trail, but I couldn't resist taking this picture. In this outfit, I may be addressed as "Cromilar 23, VoidMaster."
