Hi Alex, thank you for this excellent plugin, huge potential.
I have moved a blog over to a new WP theme and have needed to regenerate all images. The success of Regenerate Thumbnails plugin on my cheap hosting with lots of images varies, freezing between 0-50%, different each time.
What would be very useful when running the plugin would be the display of a detailed success/error log - that would really help working out what is causing the problems, what images have been regenerated etc.
Also when you run Regenerate Thumbnails numerous times, does it begin 'from the start' again?
I'm looking into better hosting but for now I am downloading all image content in my blog and will work through all the files, looking for those that haven't been regenerated and doing that myself then re-uploading.