I've been here several times now, and I have to say, it is an amazing and new experience. It doesn't compare at all to regular movies. I do recommend, however, to be sure to choose movies that are high on visuals- Charlie and the Chocolate Factory, for example, was absolutely amazing on Imax. Movies that are high on dialogue and low on visuals wouldn't be so good (does anyone actually need to see the actor's tonsils when he delivers his lines?).
As for the Regal Cinema Imax, the place is very clean and well-maintained- probably the cleanest and neatest in all of Nashville. A good movie-going experience. Avoid opening night though, as the place is PACKED.