I've seen the film now three times: HFR 3D, 2D, then IMAX 3D. I hugely preferred the HFR 3D. In fact, I was annoyed with the IMAX presentation for being so blurry, and I certainly had new hatred for the conversion on things like the Star Trek preview and the MoS trailer.
The film in 2D is fine, looks just like LotR.
I'm completely sold on HFR, and this from a guy who wasn't going to see it in HFR at all (I got invited to an advance screening). The only reason to go to an IMAX screening instead of HFR would be the quality of the sound, so IMAX HFR is probably the way to go.
I can't wait until more films are made this way. I hope the ridiculous anti-HFR backlash isn't going to keep the Star Wars sequels from being shot in the format. It really is the way to go if you want to do 3D.