By the way, from volunteering in the ER, I got a DO to write me a killer letter. I never shadowed him, and he was hardly my best buddy or anything, but I was around a lot, and he definitely knew who I was. I just came up to him, and asked, and he had no problem with it. Just a nice guy who was more than willing to help.
At least I think it was a good letter. I never saw it, but it was a couple pages long, according to Interfolio.
I never formally shadowed anyone, but volunteering in a busy trauma center, surrounded by DO's, seems to be a far better way to go if that is an option for you.
Actually, I know quite a few DO's from there, and several of them would have bent over backwards to help me, or so it seemed. I think that I was able to show them more about myself than I would have during any kind of formal shadowing. They got to see me working and helping out, rather than just following them around.
Just my opinion. Nothing better than a situation where you can be around a bunch of docs.