Be aware that this plugin inserts a link to greatwordpressthemes.com surreptitiously (i.e. without permission) to your blog upon plugin activation - so if you unexpectedly see this appear within your links/blogroll then you'll know where it's coming from.
I don't think anyone (me included) would begrudge plugin authors a little financial support in return for their efforts, but in my opinion this is not the way to do it and taints an otherwise good plugin. Sloppy.