With Transformers: Revenge of the Fallen coming to IMAX in just two weeks with full IMAX 70mm CGI effects and 1:1 ratio for some Transformers in select scenes, it seems important to cover that there is now actually two different IMAX screens out there and you might want to check which your theatre has before deciding if paying that extra price for tickets is worth it to you.
IMAX is a 76x97 feet screen while minIMAX (not official name) is 28x58 feet, a 4.5x difference in sizes (left pic). Which size is used depends on the theatre but if your theatre was recently retrofitted to an IMAX screen then chances are it is the smaller one. However, many new theatres are using minIMAX due to reduction in costs that the smaller size and space requirements. After all a real IMAX screen requires a 7-8 story room. Both sizes still use the detailed 70mm image format, the sound is usually the same and for most the visual experience will be about the same.
How can they be the same visual experience? By simply moving the screen closer to you (right image). The result is the optical illusion of a huge screen that takes over your entire visual range.
Theatres do not advertise if they are IMAX or minIMAX so click here to determine which your theatre is. As for whether minIMAX is worth the extra cost, it comes down to personal preference. Only way to find out is to check it out yourself and come to your own conclusions.
Even if you decide minIMAX isn't worth the extra charge, just remember a question you should ask yourself - is the four extra IMAX scenes in Revenge of the Fallen worth that extra price of admission?
The sad fact is minIMAX is probably the future of the format, so your options are probably limited but at least you go into the decision informed rather then feeling tricked. To get more detailed explanation, check out Consumerist and Gizmodo.
Update: It turns out that things are not as simple as two IMAX formats. There are actually three with various screen sizes, aspect ratios and so forth. Below is an explanation from "Boo" about that.
I would like to clarify some of the information in your post as I am a Projection Service technician (not a projectionist, I repair and install all the projection equipment) for one of the nation's largest chains. The difference between IMAX and miniIMAX is not just screen size (the dimensions of which can vary greatly from what you posted; my local screen is 60h x 80w). The projection technology is actually totally different. True IMAX theaters use a proprietary 70mm film that orients the image horizontally on the frame allowing for a film frame that is 4 times larger than 35mm frames. In addition to the projectors use 15,000 watt lamps. The so called miniIMAX theaters are actually Digital IMAX theaters running 2 Christie 2K (meaning the resolution is 2048 x 1080) DLP projectors each using only 6,000 watt lamps. The reason the screens are so much smaller comes down to 2 very important factors, the first is that the digital projectors cannot light screens with the size and distance from the projector of traditional IMAX, the second is that IMAX doesn't want to spend the money to build the huge houses so they are retrofitting existing houses, any retrofit will not be true IMAX.For a list of the number of different file formats and explanations click here (thanks Tiezane). As you can see, it can get complicated for the average laymen. My advice remains the same - check it out for yourself to decide if it’s good enough for your extra dollars.
Now for the most important detail, IMAX film is of a different aspect ratio than traditional 35mm (and by proxy digital) movies. True IMAX is a 1.44:1 aspect ratio where "Flat" 1.85:1 and "Scope" films are 2.39:1. If Bay is pulling a "Dark Knight" then the film will run at 2.39:1 and expand to 1.44:1 for the special IMAX scenes. The digital projectors CANNOT do this and will only expand to 1.85:1. I don't yet know if the Digital IMAX version of the film will have the same content as the 70mm IMAX or if it will be the same as the 35mm print, but it will certainly have been altered to deal with the constraints of digital if it is the same content as the true IMAX print.
Another update: Click here for another list of IMAX theaters for the US, the ones in green is the real IMAX.