Personally, on Quantum I did iFly twice because it is SO much fun. I could easily spend lots of time by the flowrider too - both participating and watching! When I'm done on any of my Royal Caribbean trips I often find myself at the last day thinking "gee, we didn't go do THIS yet". There will also be other activities on the cruise compasses and remember that there are separate compasses and additional activities for the younger groups.
As someone else said, it really depends on your desires. For me, it's important to also schedule some time to sit and people watch, look out over the ocean, or just sit and talk or maybe pull out a game. As with any vacation, it is what you make of it. If you're there to make memories I think you'll be ok.