Proper would be a film that's actually been shot in the IMAX format with 65mm film stock that's printed onto 70mm for projection.
Aside from the films which are specifically designed for IMAX, most are few and far between. One recent example that most are aware of is the scene in the Dark Knight, but that's just the one scene.
Basically, 70mm is huge, it's almost 10k of resolution which dwarfs 35mm which is typically 2k.
With films that aren't shot in IMAX format, they are simply 35mm images which are being upresed to IMAX, you can often spot the detrimental effect this has.
The fact that so many IMAX branded smaller cinemas has popped up hasn't helped things much. So In my opinion there is no benefit at all to seeing something in IMAX unless it's actually IMAX, just got to a regular screen otherwise.