Faux IMAX handles aspect ratio the same way as regular IMAX -- there are black bars to accommodate for frame size. The faux IMAX screen, however, is not 1.33:1 as a regular IMAX is... it's usually closer to 1.5 depending on the build. As a result, there may be some croppage on movies shot specifically for IMAX in 1.33:1 (like, say, "Under the Sea 3D"), but all "normal movies" are in the accurate aspect ratio. The one exception I can think of is I saw "Madagascar 2" at a faux IMAX and it fit the full screen, but I'm guessing that this croppage was done specifically and artistically as a part of the IMAX DRM blowup...
Faux IMAX is actually pretty good for 3D. It preserves brightness in a way that REAL D doesn't. The big problem with the technology is the dual 2k digital projectors on IMAX just don't match 35mm or 4k digital projection quality, in my book. Also, the frame size is TINY for 2.35 films as there are huge black bars on the top and bottom. It's not even the size of a big Scope screen in a regular auditorium.
As a result, there are only 3 reasons to see a movie on a faux IMAX: A) it's in 3D, B) it's 1.85, not 2.35, and you want to get a huge frame, or C) it's animated and/or has never touched film at any point in its development, so digital actually does preserve some quality.
I say "Avatar" is a go for faux IMAX if the real thing isn't readily available because it meets ALL THREE of the above criteria. Not only is Cameron a big proponent of digital, presumably bringing out the best in the technology, he also is using 1.85:1 for the 3D versions of the film so you'll get a nice and big picture. There will not be any croppage, unless there was some done specifically in the DRM blowup to fill the frame (which, knowing Cameron, would have clearly been done with director's approval). You will see the film as Cameron intended it. In fact, he even matted the movie down to 2.35 for the 2D version because he likes that better for 2D.
Also a plus for any type of IMAX: no aspect ratio confusion. Because this movie is 2.35 on the 2D version and 1.85 on the 3D version, there will undoubtedly be many projection screw-ups on normal digital projectors. I predict that many projectionists will see the 2.35 ratio listed on their sheet for the 35mm version and not check the 3D sheets, hence presenting the digital versions in the same 2.35 ratio. Instead of stretching the picture, as a film projector would, digital projectors just "zoom" to fit the ratio set by the projectionist and unknowing audiences often just sit by watching a cropped film. While this is all entirely speculation on my part -- I'm sure that Fox will make a big deal of the difference on the notes that come with each print -- I fear it will happen an awful lot. To the best of my knowledge, IMAX -- because there's no masking -- doesn't require any setting of the format. Then again, maybe my faith in the projectionists at big theaters is too low due to bad experiences in the past, both as a patron and an employee... just a word of caution.
If there's ever been a time I felt comfortable dropping $16 on a faux IMAX ticket, it's "Avatar".