Grammarly

Yesterday I came across some software similar to Microsoft's Spellck.  The basic program is free to use, so I gave it a try.  It found far more mistakes in punctuation and grammar than Spellck although some of the corrections it wanted me to make were still wrong.
Most of the mistakes it found were in punctuation.  I imagine that most writers miss some punctuation as their fingers fly over the keys at 50 or more words per minute.  So I decided that I really like the basic software.  Here is my question.  The software says that the premium version found many more errors in my writing.  It does, however, not give an example of what those errors might be.
My question is, has anyone used the premium version of Grammarly, and if so was it worth upgrading?  What kinds of errors does the premium version find that the basic version does not?