As you can see, traditional dimensions for a genuine IMAX screen are 76' x 97', whereas these newfangled ones are 28' x 58' or thereabouts. Just as important, faux-IMAX screens use dual digital projectors rather than projecting 70-mm film, per true IMAX standards.
None of this would be a huge deal, however, if the two systems and their respective differences were marketed as such. But they're not. Most moviegoers either don't know or don't care about the marked disparity -- if they did, it's unlikely they'd be willing to pay the same price for the one as they do for the other -- and IMAX is in no rush to change that.
Have you ever watched a high-definition channel on a non-HD TV and noticed how unnatural and even bad it looks? The same is often the case here. Colors don't pop, detail isn't as crisp, and many point to what's called the screen-door effect: a distracting grid that occurs when the lines separating individual pixels become visible.
Given that the whole point of IMAX is to creative as immersive an experience as possible, with company reps even using that argument as justification for the huge disparity in screen size, these are all significant issues. This is truer than ever in the case of recent films like Transformers: Revenge of the Fallen, Mission: Impossible -- Ghost Protocol and, this week, The Dark Knight Rises, all of which have shot a number of scenes with IMAX cameras (over an hour in the case of TDKR). Christopher Nolan himself cares about this quite a bit, which is why the six-minute prologue he shot was only shown on 44 screens last December.
There's good news and bad news for L.A. Batman enthusiasts. The good news is that we have two legit IMAX locations to choose from (three if you count the California Science Center, which specializes in nature documentaries and the like): Universal CityWalk and Rave 18 at the Howard Hughes Center. (Personally I'm partial to the latter, but I leave that to you.) The bad news is that every single IMAX screening of The Dark Knight Rises at both theaters is sold out for the foreseeable future -- the midnight showings at CityWalk sold out in January, if that's any indication of how long you may have to wait.
But if you're stressed about where and when to see the last Batman movie, don't be: they'll probably reboot the franchise in a couple years anyway.