I have all three, and yes I do think they are different enough, in intensity and tenor, to justify getting them. Interval Max (the original; on the same DVD as Rhythmic Step and MIC) has extremely uniform intervals with very little variation in the drills during the blasts themselves. Pure training and sweat, very very basic choreo. I-Max 2 has more of a "fun-factor" built into both the aerobic segments and the intensity blasts, and there are more vocal selections in the music. It is less intense, IMHO, than Interval Max or I-Max 3. Plus, I-Max 2 comes on the same DVD as Cardio & Weights, which is a much easier workout but comes with an awesome ab/core segment.
A-Jock