The Lego Mythica area opened last month.
I went to Legoland Friday evening for an Annual Passholder event (was very quiet, but largely because of the downpour of rain all day).
Flight of the Sky Lion was my first flying theatre and it was pretty decent. A very fast paced experience for the film. At times it would have been nice to just take things in a bit more a 'fly' around (which I hear other such rides do a bit more?). I sometimes struggle with a bit of a motion sickness of some motion simulators, but this was fine, though I felt a bit queazy about 3/4 of the way through.
My biggest pet peeve is the fact you can see the edge of the screen at the top, and the roof as a result. It's particularly jarring given the screen extends to the floor, so looking down to fine, but looking towards to top is a bit awkward. Not sure if this is a common problem with flying theatres though..? The queue line, as already pointed out, is pretty dreadful. I hate to think what it's like on a hot day..
The Mythica land is solid as well. Well presented and nice theming, plus bringing attention to a previously quieter part of the park. Definitely think it's much better than the originally planned Lego Movie World would have been too.
All in all, an impressive addition to the park. Would recommend giving a shot.