In my personal opinion, one cannot really comment on a show if you've only seen half. I'm not saying that things tend to get monumentally better if you're sitting through a train wreck, but the idea of seeing half a show is so bizarre to me. Even if it's awful, I'd rather see the entire production and be able to speak to what I saw than ditch.
I've only ever left before a show was over once, and that was on two comp tickets to "Rain". After the first 10 minutes, we both had absolutely no interest in sitting through the rest, and since we hadn't invested much time or money into it, and were sitting on the outside aisle, we snuck out and went to Chipotle instead. Much better use of time.
"Get mad, then get over it." - Colin Powell