The writing is a bit dated (wonder why that is), but there are some good stories in the other volumes. Superman's cast of supporting characters (Cat, Jimmy, Perry, and Maggie Sawyer especially, among others) are developed and established better than any writer has ever done before or after, IMO. Plus there are the first appearances of some cool villains, like Metallo, Skyhook, Prankster, and Toyman.
I think it's in issue #9 (not sure which volume that would be in) where there's a short story about Lex at a restaurant toying with a random waitress and completely screwing her over, showing just how evil the (then) new Lex Luthor was, even just in his spare time. I don't have the comic with me right at the moment, so I'm not sure exactly which issue it's in.
The stories are like MOS, meaning you shouldn't expect every plot thread to be tied up by the end of each arc. So yeah, just keep in mind that this was a time when they were working to establish elements in Superman's new universe and purposely set up some unresolved plot points to use in the future, much like a television series. Personally, that's the way I compare MOS with later stories like Birthright that come to a satisfying conclusion; MOS is more like an ongoing TV series (never intending to end), while Birthright, Godfall, etc, are more akin to movies. That's just me, though.