There is a nice writeup about this at The Indian Blogger (can't add the address because I haven't reached my first 5 posts) which you may want to read. According to the writer, there are 2 types of article rewriting software:

• those that provide a list of synonyms and similar phrases for the exact words/phrases used in the article.
• those which provide a point & click interface where you load an article and get a brand new article just by clicking a button. Though very fast, they have a tendency to completely distort the meaning of the sentences used in the article and render it ineffective.

The writer's conclusion is this: "I found that all these softwares (particularly those recommended by so called Internet Gurus) are a pure waste of time and money."

Personally, I do not think that any software today can yet replace human brains. If you have tried any translation software or Shvoong's summarizer, you will know what I mean.