Slate has an interesting article about blogs written by tv characters. They conclude that "most of these blogs kind of stink."
I don't know if I agree. In fact, browsing through my archives, I saw that I mentioned two that I like before: Barney from How I Met Your Mother and Dwight from The Office (Creed's blog is kind of creepy, though).
I think that, to a very large extent, whether you like the blog is going to depend on whether you like the show and the character. I think Barney and Dwight are hilariously entertaining fictional characters, therefore it's likely that I would be entertained by fictional blogs written from their fictional perspectives.
But I agree that blogs written from the perspective of a tv character don't always work. Even though I watch Grey's Anatomy, I think the nurse's blog is a waste of time - it's not from the perspective of a main character, and all it seems to do is recap the episodes. It doesn't add anything.
Now that I think about it, I think that's the difference between any promotional website for a show or a movie or whatever -- does it capture my attention, and does it add to my enjoyment of whatever it's promoting? Like when I first started watching Lost, I would go to the various websites to try to figure out what stuff meant, which added to my enjoyment of the show. Or maybe added to my confusion and frustration. Bad example. Whatever.