I've seen a load of 3D films and to be honest, the 3D doesn't add much and the dark picture detracts more. Its a gimick, as is Imax.
OK - I realise Imax is better quality but by the time you factor in how far back you have to sit, there is no benefit. You've paid all that money for nothing.
Your right. The only noticeble differnece for me with Imax, and it may just be the theater itself, is the sound is so much louder and more crisp. I find this funny in that you go to Imax for the view, but the sound is what is actually better.