It didn't seem like christian propaganda to me, at least no more so than Lucifer or Good Omens or Exorcist. Most of the angels are dickheaded assholes, and a few of the demons are halfway decent. God is a bit of a jerk, and Satan is moderately reasonable in a cold-logic kill everything kind of way.
Not only that, they left Jesus out altogether.
That's an understatement.
As an example, I did like the shows Buffy the Vampire Slayer and Lucifer, even though they would seem to touch on similar subject matter, but I felt that the writing and charm of the characters well made up for it. How does Supernatural compare to these shows in quality of writing and plotting?
Dean is in it for the first few seasons, then his character becomes a bit less involved. Probably so he could kill demons or something. Gilmore Girls was a good show, but I could only watch it through the first run. The main characters are self destructive monsters and it is much harder to emphasize with them the second go around.
Once you get past the first season, I'd say it's marginally better than Buffy, but not as good as Lucifer. It's sillier than Lucifer, but not as packed with teenage girl angst. The characters are good, although Sam annoys me most of the time. The writing is good (after S1), I'd say on par with Grimm.