Adam Dachis from Lifehacker recently put Dropzone through its paces:
The software we like to feature is generally free or close to it, but Dropzone turned out to be so much more capable than initially expected that it was worth an in-depth look.
You can read his full review here. I definitely recommend you watch the video he made (above) as it provides an excellent introduction to the app and how to integrate it into your workflow.