In short: D+.
More thoroughly...I walked out of the film reasonably pleased, but then my brain re-engaged (presumably switching itself off in self-defense upon realizing I was walking into a Zack Snyder film) and immediately
I started finding faults. After the opening scenes on Krypton, which are well-done, Man of Steel quickly loses all sense of direction. The casting is great, but they're given nothing to work with—that Adams and Cavill manage as much chemistry as they do is a testament to their skill, not having anything to do with writing or directing. The effects and visuals are gorgeous, but the film is so leached of color that, among other things, Superman's suit might as well be dark gray, and that's in 2D. The childhood sequences are acceptable, Pa Kent's character aside (don't even get me started...), but the script or editing necessitating those be flashbacks absolutely ruins the pacing of the film as a whole. I could even buy the big ending, which I know many fans of Superman might not be able to...if they had just set it up better. As it is? Well, it could have been worse, I guess. In fact, virtually everything positive I can say about the film comes with "but [something negative that all but invalidates the positive]". About the only thing I can recommend wholeheartedly is Hans Zimmer's score (and even that's not really true, coming with the caveat of "if you're a fan of Hans Zimmer").
In the end, Man of Steel is sit-throughable. It is everything you would expect from a Zack Snyder superhero film with a weak script—it is style over substance, as shallow as a coat of varnish, diverting and vaguely entertaining but ultimately empty of anything but spectacle. It's in no way a Superman movie, and the word "disappointing" doesn't even begin to describe it.