Judging from the photos, it looks like the best time to visit was when they had the Chihuly installation. There are a few pieces still there and they are absolutely breathtaking. It was cool seeing all the different types of desert plants, and I'm sure it's even better if you're able to time your visit for when they are in bloom.
One of the coolest things was learning what it looks like inside a cactus. I would have loved to see a plaque or some interactive exhibit with a cross-section of a cactus that showed how everything works.
All in all, I think it's a great stop if you're visiting Phoenix.