Ok, I've posted this in other threads on this topic, but I'm going to say it again to be the voice of dissent:
I would actually say that you SHOULD read the play first. It's what I did, and I'm really glad. Thing is that the writing IS pretty mediocre, like others said. But because I had already read the script when I saw the play, I had already gotten used to the bad dialogue, and I was just able to focus on the beautiful stagecraft. If you haven't read the script, you may be distracted by the bad writing (because it will be new to you) and it may taint your overall experience.
Also, the script is written in such a way that it's designed to pique your curiosity - which is fun. The stage directions say WHAT magical things happen, but they don't say HOW. So as I was reading it, I kept wondering "ooh! I wonder how they're going to do that!" and I got more excited to see it onstage.