Illuminations is our favorite show - we LOVE the music from it.
That being said, we like Fantasmic, but don't think it's great. We always see it once, and a couple people in our family go to see it twice, but personally, I think that they do WAY too much on the water screens. We've seen it from nearly every section of the theater and personally don't think it makes that big of a difference where you sit (although I know others disagree).
Granted, we're a family in which the youngest person is 16, so we generally bypass shows with characters (and is probably why we LOVE Illuminations and see that 3-4 times each trip). Fantasmic isn't ideally aimed at us (although ideally Disney's attractions are aimed at all ages).
So, to conclude this long-winded post - We like Fantasmic, but aren't thrilled by it. We see it once each trip and that's enough.
RyMickey