15 years of episodes create a bit of a mixed bag in this series. While there are some stinkers in here and some of the seasonal arcs are better than others, the average quality of the episodes and the presence of some real gems in the episode list make this one well worth the watch.
I have watched this straight through twice and watched the first 10 seasons or so another two times. There is some excellent character development. There are really too many to list in a single review but even some of the roles that lasted only a few episodes were quite entertaining. A good example of one of these was Ash from the Roadhouse. Also Bella was extremely good in her role. The long-term characters like Bobby and Crowley along with Lucifer were just great.
If you like the genre of "things that go bump in the night" this is a great series. You have to stretch your imagination to take in demons and angels and werewolves and vampires but if you can do it, this is good stuff. For me personally the most difficult thing to believe was how the random female extras, especially in the early seasons, are all 7+ on the man scale of 1-10. An uncanny supernatural babe-fest.