The best possible monsters vs. robots movie imaginable. Also, it is a monsters vs. robots movie.
Or, as one reviewer put it: "This is THE robots vs. monsters movie you wanted Guillermo del Toro to make... if you wanted him to make a robots vs. monsters movie"
Film fan boys should see it. It certainly appeals to the 14 year old boy in one. It is creative (within the context) and it is a lesson in how to make a movie like this. eg. All of the fighting sequences make sense -- you always know where each robot is, where each monster is, where they are in relation to one another, what has just happened, etc. It isn't like the other crap where things just go flying and appear out of nowhere and disappear inexplicably, etc. In certain regards, while I don't think it is quite as good, it evokes Spielberg's Jurassic Park in that sense; within its own self-contained world, what is going on makes sense and is consistent.
Most fanboy-ish, of course, is that Guillermo del Toro directed (and co-wrote... in this case I think meaning lots of script doctoring) it. Since Pan's Labyrinth he has been just about the most sought after director in the industry. (BTW: If you haven't seen Pan's Labyrinth put that next in your queue... just a freakishly creative, wonderful and compelling movie.) And his artistic and creative vision is already legendary. (2015/2016 cannot arrive fast enough for me to see his dark, gothic, "definitive" live action re-telling of Beauty and the Beast.)
Overall, I would give the movie a flat B, but with the proviso that it is interesting, creative, and well executed enough that I would encourage people to see it. Try to see it in Imax, but regular 2D is great, too. (Apparently, it is too dark in regular 3D.) B