A proper large format 15/70 IMAX theater is definitely worth it given you arrive early to get the better seats - especially for 3D films where the 3D glasses tend to make the screen look smaller.
Sadly, the IMAX CEO says screen size does not matter [1] and thus the majority of them are being replaced by smaller digital screens in multiplexes. lfexaminer.com has a great list of IMAX theaters and their formats [2].
Regarding the 3D experience, Real3D's circular polarization is significantly better than IMAX's more traditional polarization filters. The large screen format serves well for immersion, but the employed 3D tech detracts from the overall experience so much that I'd rather watch the smaller format screens.
In my city, we have a cinema that has a 18m by 8m screen and uses 4K projectors with reald 3D. It’s still an amazing quality, and faaar cheaper than IMAX. (About 3 times cheaper). And they clean the room after every showing, so the floor, although carpet, is always clean, you get reserved seats, and the popcorn is fresh.
IMAX, especially as current IMAX Digital theaters have similar screen sizes and also only 2.8K (two 2K projectors on the same screen), just isn’t really worth it anymore.
Sadly, the IMAX CEO says screen size does not matter [1] and thus the majority of them are being replaced by smaller digital screens in multiplexes. lfexaminer.com has a great list of IMAX theaters and their formats [2].
[1] http://www.wired.com/2009/05/imax-ceo-screen-size-isnt-every...
[2] http://www.lfexaminer.com/theaUSA.htm