I guess whether you like this episode depends a lot on whether or not you think Becky is funny. I do, I like both the character and the concept of a meta character, so I was really tickled by the whole thing. Almost all the comments I've seen on Twitter also loved the episode, so I guess I'm not alone.
Let me start by saying there's no way they are trying to "wean us off Sam and Dean" and replace them with Becky and Garth. Supernatural cannot exist without Sam and Dean: they know that as well as we do.
They're just giving Jensen and Jared a bit of a breather. The boys work 14-20 hour days with very short weekends because Fridays always turn into what they call "Fraturdays" on the Supernatural set. When there are only two leads on a one-hour show, the schedule is grueling, and the boys have been at it for seven years now. It's not unusual for shows, especially in the later years, to deliberately write scripts that give the actors a lighter workload once in a while. Doctor Who even has regular "Doctor-light" episodes in which the Doctor makes no more than a cameo. So no need to worry, Becky and Garth will never be more than recurring characters. (Actually, we've probably seen the last of Becky, the way they ended it. Sam made it pretty clear he didn't want to see her again.)
I liked how they took Becky's crush on Sam and turned it not only into an episode-long joke, but also into a cool case file. As I have said before, I love the detective work that goes into solving supernatural mysteries, and I love it when the mystery gradually unfolds and I don't know what's really going on until the end. This certainly fit the bill. I didn't figure out it was a cheating crossroads demon with an accomplice until the red eyes were revealed. The fact it was the accomplice doing the killing threw me off the scent as he didn't seem to have any connection to Guy. It seemed like Guy was working alone and was maybe a witch, as he was dealing love potion--which turned out just to be the bait to lure Becky in for the deal. And I loved Dean and Garth figuring out it was the domineering wife who made a deal, and not the CEO.
I agree it seemed as though Garth was being introduced to us a new recurring character, rather than being used as a one-off guest star. Everyone I saw tweeting about it were clamoring for more Garth, and DJ Qualls (the actor) and Jim Michaels (executive producer) both hinted we might be seeing more of him. I think that was the point, to leave us wanting more. And when Dean said "you don't suck" and Garth answered "that's the nicest thing anyone has ever said to me" I didn't want that to be the last we ever saw of him. He's adorable.