I have been a fan since the first Ghostfacers episode, but my wife recently started watching Supernatural with me. We started at the very beginning, and are about a third of the way through the first season. She seems to be enjoying it so far. This pleases me to no end.
"Carry on Wayward Son" by Kansas is the unofficial theme song for the show, catching viewers up right before the start of a new season. I love that song.
The early episodes are a little hokey, especially with the necessary explanations of monsters and weapons to the audience by one character over-explaining it to another. This happens a lot with just about every exchange between Sam and Dean. It's a necessary evil, and one that I can't quite get past.
If you haven't watched the show before, give it a shot. Nearly any episode in season one is a good place to start, at least until the last five or so episodes. It's definitely more than just Buffy the Vampire Slayer meets The Hardy Boys, although I'm pretty sure that's why she's enjoying it. Her celebrity crush on Jensen Ackles probably doesn't hurt that either.