I got one recently at Best Buy too, and I did get the three year protection plan for the peace of mind. The clerk told me they'd replace it for pretty much anything. But, after reading Itdonnedonme's Consumer Reports link, it sounds like maybe it was a mistake. Sounds like I was a sucker. I'll think twice or thrice or whatever next big purchase I make, if ever. Based upon that, it sounds like you're probably good not buying it. But I'm kind of an airhead, so what do I know?
Don't know if it's worth it or not, or whether the Canon support would be good enough without it. I've pretty much only heard good things about Canon replacing cameras etc. But, contrary to what GA said, I have only good things to say about Best Buy, as well. It's been one of my favorite stores. Although, I've never had to ask them to replace a camera before, knock on wood. I do like the idea of just going to my local BB if something calamitous happens. Again, not an argument for buying it. I'd tend to listen to Consumer Reports. Just saying that BB in general has been good to me for returns, for example. I tend to believe them too. But of course, sadly, they've been having trouble competing with the Amazons of the world, so what does the future hold?
Something that gave me pause was my wondering if Canon might discontinue the 5DII before those three years are up. Seems quite possible. Didn't they discontinue the T2i? So, what if your camera needs replacing after it's no longer being manufactured etc? What would be BB do? I dunno.
Not really an anwer, sorry. Good luck.