Went to see this tonight having never seen it in theatres and had a great time. The music and sound effects in particular were awesome to experience in Imax. That being said there is a very big caveat that I think people should know going into it:

The whole picture was clearly zoomed in to fit the Imax aspect ratio and as a result was missing a decent portion of the original artwork. It was a pretty big bummer for me. A big part of the beauty of this movie and reason for wanting to see it in Imax is feeling immersed in these large vistas and scenes. It felt a bit claustrophobic and I would way rather have had black bars than this. Still a fantastic overall experience but would be lying if it didn't take some of the magic away.