Looking at the examples they give, it looks like a good idea. However, many of the error they detect could have been detected by anyone with a good knowledge of English, which comes merely by reading a lot of books, and not necessarily by getting an English Degree.
So on this page, one of the errors they give is:
Good things come to them who waits.
Now I saw the error immediately, without having to look their solution. Another example is:
My birthday was June 31, 1986.
The date is incorrect - I couldn't guess this one.
So overall, not a bad software, but it might be less useful to those who already are in the habit of reading. The key point is the price - do you think $55 is a fair price?
They have a ten day trial- you can try that to see how it works in practice. Of course, you will need to have a fairly long piece of work to see how good the software is in practice.
As to which version you buy, if you normally use Ms Word, then you should buy the plug-in. If you use an alternative like Open Office, or any other free tool, then buy the stand alone version.
Just giving my personal opinion, I'm not too sure of software that claims to replace what a human can do, at least in creative fields (remember Clippy?). So be sure to use the trial before you buy the full version, and do share your results here.
Edit: Ok, I actually tried the software, and it was worse than I thought. It looked like a 1990's shareware program designed by a student. The UI was weird- you have to press 2 weird sounding buttons like draft and usage to get your analysis. The analysis itself is in a small non-resizeable window.
Even if you ignore the clunky UI, the actual program isn't that great. I entered about 1200 words from my draft, and it threw out hundreds of suggestions, most of them useless. Eg, in one case it told me the word 'assistance' was pretentious (really?) and I should replace it with help. But assistance was the right word in that scene. Then it wanted to replace hoarder with boarder.
Now you can say, the writer should be able to take what he/she wants from the output, ignoring what they don't like. All well. Except that it was throwing up hundreds of suggestions for just 1200 words. If I entered my whole book, I would be bogged down for weeks trying to understand the software's cryptic messages.
And finally, the price. currently, it is $55, which is more than even Scrivener. I'm not sure this software justifies the high price for the value it offers.