Having recently moved to Castaic: I have lived in many places in Southern California and I can tell you that being in and around Valencia and Santa Clarita (both a few minutes from me), living in the Santa Clarita Valley is the best choice I ever made.
Valencia and Stevenson's ranch, generally speaking, have some of the nicest places, but just like anywhere, you have to really drive around and spend some time, look at houses, look IN them, drive around neighborhoods, etc. Santa Clarita, Newhall and Canyon Country have their share of "rougher" patches.
In general I have never been happier living anywhere. Yes, the commute can sometimes be frustrating so if you are sensitive about that (I am not), do the drive a number of times to get an idea of it. I think the commute can be a point of buyer's remorse for some.
But it's worth it for all the friendly people, happy families, hard working people, clean streets, prospering businesses, it's one of the few truly beautiful and friendly places left, without being a mega-millionaire.