I would echo Matthews feelings about Deadliest Warriors.
It's a neat idea and I wish somebody would do it a bit more seriously (along the lines of Mythbusters) but sadly it's really more of a 'cheap thrill'.. That said I really wanted to like it, I do watch it, it's a bit of a guilty pleasure, I enjoy seeing all the test-cutting and test-shooting, and they have even occasionally demonstrated some useful memes like the (to most people) surprising idea that ancient armor can actually stop bullets, or seeing a Sikh chakrum used successfully to cut through a big hunk of meat which is something I'd never seen before.
My assessment boils down like this, each episode of that show puts out a certain number of memes or ideas, lets say for sake of argument 10 memes per show. Of that number, typically 5 or 6 are at least partly wrong, and 3 or 4 are outright misinformation. It varies a lot depending which advocates they get for each side. For example, in each episode there is usually at least one weapon which is totally fake, like a 30 pound club, and most of the others are really cheap poorly made replicas. On one episode they might use halfway decent riveted mail for a test, on the very next episode they might use butted mail which did not exist in an Historical European context (except in modern Renaissance Faires).
These 'experts' on the show are not actual academic or even amateur Historical Martial Arts experts but appear to be low level stunt or fight coordinators who specialize in this or that genre, and probably do most of their work for Sci Fi Channel. Some are better than others. So no, I personally wouldn't recommend it as a direct source for an historical campaign.
That said, there is a pretty WIDE gap between historical reality and the version of it portrayed in most RPGs. So maybe you could argue D.W. is somewhere in between a Teifling warrrior carrying a 9 foot spiked fanged bat winged swod and say, a real 16th Century Landsknecht.
I would say if you wanted to use Deadliest warrior as a source, use it as a point of departure. If you see an archtype on D.W. that you hadn't heard of in an RPG before and are interested in, use some other sources for further research. I'd reccomend those Osprey military books as a good starting point, they are probably 80% or 90% accurate (roughly the opposite ratio of D.W.). And you can find an Osprey Military book for practically every single kind of warrior who ever existed in history in any part of the world, from an Aztec Jaguar Warrior to a Knight Templar.
[ame="http://www.amazon.com/Aztec-Mixtec-Zapotec-Armies-Men-at-Arms/dp/1855321599"]Amazon.com: "Aztec, Mixtec and Zapotec Armies" (Men-at-Arms) (9781855321595): John Pohl, Angus McBride: Books[/ame]
[ame="http://www.amazon.com/Knight-Templar-1120-1312-Warrior-Nicholson/dp/1841766704/ref=sr_1_1?s=books&ie=UTF8&qid=1279749216&sr=1-1"]Amazon.com: Knight Templar 1120-1312 (Warrior) (9781841766706):…[/ame]
And some of them are even availble online for free on google books and other sources.
There are also many good videos out there such as Mike Loades "Weapons that Made Britain" and (though Matthew would disagree with me) Terry Jones various BBC documentaries, particularly his "Barbarians" and "Medieval Lives" series which I thought was quite good.
[ame="http://www.youtube.com/watch?v=qEMwcSGauY8"]YouTube - Weapons that Made Britain: The Sword (Part 1)[/ame]
[ame="http://www.youtube.com/watch?v=Y-BQpfeKtWE&feature=related"]YouTube - Terry Jones' Medieval Lives- Outlaw 1[/ame]
[ame="http://www.youtube.com/watch?v=Ekbzbd0gOLs"]YouTube - Terry Jones' Barbarians - "Savage Goths"[/ame]
G.