BTW, spoiler alert.
Among fans, I think, this season is Supernatural's most controversial. Despite some strong stand alone episodes and an ambitious season long story arc, the whole ends up being less than than the sum of the parts.
Season 6 was unfocused - with multiple, partly developed villains and little sense of forward momentum.
Season 7 was, obviously, intended to be a course correction with the introduction of the Leviathan as a season long villain. Conceptually, the Leviathan are amazing - villains who have been locked away for all time are unleashed on the world; they can look like anyone and they can smoothly fit into society. They have a master plan to enslave humanity. It's a cool idea. In practice, it wasn't effective. Despite some creepy, vaguely disturbing acting by James Patrick Stuart as their leader, the Leviathan threat somehow failed to feel "real" and was instead a complex puzzle to be solved rather than an enemy to be defeated. The final few episodes - in which Sam and Dean learn how to kill the Leviathan, collect the components of the weapon and finally manage to kill Dick Roman - make for compelling, interesting viewing. And yet . . well, Kevin Tran's frantic line to Sam "Dick's got creamer, he's going to kill all the skinny people" is an unfortunately laughable moment in an otherwise taut episode. And yet, even with all the challenges, the outcome felt too easy.
The stand alone episodes were an equally mixed bag - some such as "Of Grave Importance" deserve to be viewed several times. Others, such as "Party on Garth" aim for and then miss humor and/or quirkiness.
I think it is fair to point out that season 7 is a perfect example of ambitious story telling, and a perfect example of the creative team not quite being up to realizing their ambitious story telling. However, Season 7 sets up a number of important future plots namely the god tablets and Dean's year in purgatory; and it introduced Charlie Bradbury and Kevin Tran as characters, both of whom go one to become important (albeit in very different ways).
At the end of the day season 7 is important for Supernatural because it was pivotal - a season during which the show was struggling to find its creative footing after the 5 season long story arc (seasons 1-5). If season 7 doesn't quite work on its own terms, it does succeed in setting up seasons 8, 9 and 10 which were compelling and interesting viewing. Season 7 remains, for me, a mixed bag - better on a second viewing than it was on the first, arguably, Supernatural's most frustrating season.