At first, I thought this plugin was great. It even gives you a little sidebar widget that you can display when you're creating a post.
But I was just adding a new post to my site, and I realized some images were missing from a few recent posts. Sure enough, the Thesis custom image field was empty. I tried updating it, and the data wouldn't save. Then I noticed the permalinks were screwed up. It kept using the permalink of the next scheduled post.
After tearing my hair out for a while, I deactivated this plugin. Sure enough, problem solved.