My manager and I have been experimenting with it for some stuff, and our most recent attempt at using plan mode was a refactor to change a data structure and make some conversion code unnecessary, then delete it. The plan looked fine, but after it ran the data structure change was incomplete, most of the conversion code was still there, and it introduced several bugs by changing lines it shouldn't have touched at all. Also removed several "why" style comments and arbitrarily changed variable names to be less clear in code it otherwise didn't change.
This was the costliest one we had access to, chosen as an experiment - took $20 over almost a half hour to run.