I'm a well read grad student who's bluntly honest about all things, although I try to be most honest about myself.
While it was nice to get some background on the monsters, and the mythology, of the show, there were some fairly big flaws. For example, they say that Azazel - yellow eyes - is the demon they're looking for, they don't know who he is, but he can only be held by the Sigil of Azazel. Why are they looking at more minor demons because they have shiny - maybe yellow, as they point out - eyes instead of exploring Azazel because, well, there's a much more definite clue to who ol' Yellow Eyes is. This is very much unlike Sam and Dean.
And then there's the tense issues. At some points, it's we and I, so either Dean or Sam is writing this - I'm guessing Sam, and there are some clues, although it's never made very clear - and at some points Sam and Dean do things. Which is it? Third or first person? Choose one and stick to it.
Not to mention some of this doesn't sound like what either Sam or Dean would say, although luckily those instances were pretty rare.
I ignored the page long Latin recreations of whatever was used in the show, as I don't understand Latin, certainly not well enough to get much from those. They bored me, so I skimmed through them.
And these types of things kept bugging me while reading this. Three stars seems fair. I enjoyed it, more than it aggravated me.