Well, there are a couple things you should be aware of:
- If you take the 405, the exit you'll need to take (Sand Canyon) is currently under construction. That means taking a detour and hopping briefly on the 5 to bypass the earthmovers and workers.
- There are unusual hours of operation at the park, and they differ by type (the balloon has different hours than the museum, for example). Check before you go so you won't be disappointed.
- The website mentions that the Hot Air Balloon is open Thursday through Sunday. What it forgets to mention is that on Thursday, it's supposed to be open for groups. You can make your own group reservation in advance and you only need 12 people to do it. However, the staff was aware of the oversight, and they graciously and cheerfully fit us in with a minimum of fuss.
With all that said, I still highly recommend the OC Great Park as a family destination. It was even worth the drive from L.A.