My son was lucky enough to receive a membership for his birthday last year and we went several times taking the Light Rail from Tempe (which is also fun). Otherwise, I can imagine it can be expensive to visit the Science Center. I think if one was able to afford a membership, it'd be the way to go.
It is a really cool experience, lots of stimuli and things to do and learn about. There are many rooms and areas based on different aspects of science. And sometimes they have mini-exhibits accessible to all who pay the general admission.
I highly recommend going at least once to families with children. It is a very educational and entertaining experience.