I think, going to the Planetarium would be worthwhile if you make a package deal out of the whole Olympic garden experience. So doing the Biodome and the Insectarium with the Botanical gardens will give your trip to the Planetarium more variety and something different to do.
Just sitting back and relaxing on the nice and comfy beanbags was a great way to just hang out and look at the stars and planets. It was very relaxing. The other show was pretty educational too.
You get about 30 minutes in between the two shows. Just enough to look at the exhibits. But you would have better time spent on your iPhone browsing the internet...yes they have free wi-fi so no need for roaming if you're from out of town.