In few words Imaginaerum is a dive into a man's surreal dark dreams with superior strengths in terms of powerful images and music and some minor weaknesses in character development and acting. What may appear as another childish adventure at first quickly develops into an emotionally demanding journey dealing with very adult conflicts on a metaphorical and artistic level.
In detail Imaginaerum starts off rather associative. Really strong pictures and music (reminding me of Tim Burton movies) guide the audience from the very first second deep down the rabbit hole. The Scenery is especially impressive considering the budget of the movie. While technically being a convincing and easy show, scene jumps and plot appear rather cryptic at first. The movie succeeds at creating a rough and positively incomplete idea of the plot in your head quickly and developing that idea scene by scene throughout the whole movie without letting anything become too obvious or boring. The basic ideas appear metaphorical, yet decipherable and believable. I might add that Imaginaerum demands much (but not too much) of the viewer rationally as well as emotionally: It requires active thinking to follow the central plot and it is necessary to apply yourself to the emotions of the movie to understand the scenes. Not doing so will greatly decrease the value of the experience (and may result in questionable reviews some might have read so far).
The only downside to me is that there was unutilized potential of even deeper character development. They all started interesting but ended a little abrupt. In addition I was not totally convinced by some character acting. In some moments I got that "There is an actor playing a character"-feel which shouldn't come up. Surprisingly it didn't affect emotion and meaning of the movie too much so I can overlook that weakness easily. Having mentioned weaknesses I also want to mention that there were many great moments in terms of acting as well. For example I really enjoyed the great performance of actor Tuomas Holopainen!
Imaginaerum as a "band-movie": I don't consider myself a Nightwish fan even though I don't dislike their music. I was positively surprised that Imaginaerum, in contrast to other band movies, did not make it a 2 hours narcissistic music video. A few times the band is on screen but only adding positively to the atmosphere.
As being someone who enjoys strong images, demanding plots and artistic implementations I totally recommend this movie to my friends and to you. I advise parents to be cautious showing Imaginaerum to children, as some scenes "can be disturbing" (as other reviewers already mentioned) and the movie as a whole may be hard to understand. I personally recommend this above 12 years of age.