IMO, plan mode is pretty useless. For bug fixes and small improvements, I already know where to edit (and can do it quickly with vim-fu).
For new features, I spend a bit of time thinking, and I can usually break it down in smaller tasks that are easy to code and verify. No need to wrangle with Plan mode and a big markdown file.
I can usually get things one-shotted by that point if I bother with the agent.
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.
We reviewed the plan manually, asked it a few questions to clarify parts, and manually tweaked other parts.
I didn't catch what it was, some web dashboard that showed the cost per prompt. We could see it going up as it ran. We were just using the plan our company provided.
For new features, I spend a bit of time thinking, and I can usually break it down in smaller tasks that are easy to code and verify. No need to wrangle with Plan mode and a big markdown file.
I can usually get things one-shotted by that point if I bother with the agent.