I have used the WP Theme Manager Plugin to upload over a dozen themes from the WordPress 1.5 Theme contest. It worked like a dream. I decided to use Mike Cohen's Random Image theme for my blog--and that's when I discovered the plugin's major flaw. It uploads all the files as user "nobody." That means only "nobody" can do anything to or with them. WordPress is also "nobody," so you can edit any of the components of a theme using WordPress's theme editor. But if you want to change the image that came with a theme that you uploaded with the WP Theme Manager plugin--you're stuck. You can't ftp a new image into a folder created by the plugin, you can't overwrite an image that's already there, you can't delete an image that's already there, and you can't change the permissions on any of the files in that theme--all because they were created by user "nobody." I've been told that one can change the ownership of files if one has shell access, but my webhost doesn't provide it. I could enter a help ticket to tech support and ask them to change ownership on all those files in all those themes, and I'd find that rather embarassing. For now, I copied all the modified parts of the theme using Windows copy-and-paste, and saved the theme to my hard drive. Then using the Theme Manager plugin, I deleted the theme I wanted to use. Then using my own ftp software I uploaded the theme, so that I now have ownership of it.
Unless I discover a better way to change the file ownership than whining to tech support, I must regretfully give up on the Theme Manager plugin. I am starting this thread mostly to make other people aware of the situation.