I have driven by often, but I have only gone in once (I prefer to haunt the Gothic Quarter when in Valencia). It has an interactive museum which is pretty interesting (open), like a space-age fun house in many ways. I missed the Imax theatre (also now open), but it is probably as much fun as any Imax theatre anywhere, and the planetarium should be worth the visit if it is done yet (?). The whole compound is an overall beautiful place, nicely finished, but quite vast - a perfect place to use Kamen's Ginger personal transort device. Wear those walking shoes when visiting there. City buses are convenient to get to it, and it is a reasonable walk from the city center. If you think of it, stop in any tabacalera ("tobacco store") and buy a "Bonobus" ticket for half price bus fairs - the bus in Valencia is a great deal.
The staff is friendly and helpful (normal in Valencia), I took a handicapped friend one day and they treated him like a prince.
The bottom line is that it is very entertaining just to visit it "as is" in it's un-finished state, but check first to see which parts are open for business. it is also very interesting to simply look at it from the city bus on the way to the ocean beaches - it's huge buildings are nicely set in the extensive public gardens that rest in the old river bed (the Turia has long since been re-routed, and the city had the good sense to use the space as a garden instead of a freeway).
the complex is sited in a way that is most intriguing, without appearing to loom over the city, and is worth the visit.
One interesting note, there seems to have been a budget issue, or perhaps some oversight, as the acres of glass curtain wall of the science museum have no "built-in" cleaning system, and some parts often seems to need a good rinse.