I saw it in 3d HFR, it wasn't imax, but it was cineplex's new 'ultraAVX' thing, which has a bigger screen and surround sound, I believe. So it was almost IMAX, but not quite. I really liked it, I noticed the 48 fps right away, I though the opening scenes were really bright and looked very much like a generic fantasy marketplace, not too middle earthy. However, I didn't notice the 48 fps after that; which I think is good, because it's like HD, you notice it right away, and then you just get used to it. The 3D was also good. I've only seen one other movie in 3D, which was tintin, and that was horrible. It was really dirstracting and seemed to be cutting in and out in tintin, but the hobbit didn't have any of those problems.
There were a couple of 'arrows shot right at you' type (gimmicky) 3D moments, and other than that it just added another layer of depth to it.
Tl;dr, I would say it's worth the extra $3.