Like others, I use YouTube as a way to learn how to disassemble my Glock 23, my Kimber 1911, and my Chinese SKS.
Its also a fun way to learn about other weapons from people who own them.
I don't trust myself to do any serious mods on my guns, but can see how others would.
I also think its important to keep an eye on the pos/neg count on the videos and amount of views the video has had. I'm more inclined to trust a highly rated, highly viewed video.