We went down in March, trying to catch a launch (but it was postponed). Our twins were only 3 and a half but they really enjoyed it (though their favorite part of the day was the play area, of course!).
My son loves, loves, loves the shuttle, which is why we went.
It was worth it for us but we will probably wait until they're around 9/10 before we go back again for the whole experience (we are going to try for a launch in November).
Have your kids expressed an interest in space/NASA/shuttle launches?
If so, then this is a good time to go (as they will still be prepping shuttles for the last launch, scheduled for Feb 2011).
If they have no interest and you plan on coming down to Orlando again in the next few years, then you could save it for later.
You can always play it by ear and see how the trip goes...