This, being a parody implies the previous reading of the book being parodied to get the full enjoyment out of it. Caveat: if you haven't read Fifty Shades of Grey (you are so lucky...) DO NOT read it just so you can read this book.
As a parody it's quite funny. But it does not justify reading Fifty Shades of Grey (but then again, what does?).
I've read plenty of Twilight parodies (some incredibly funny!) and one could say Fifty Shades of Grey is, in itself, a Twilight parody - the other option being that it's a horror adaptation of a horror book. Like Stephen King writing something and handing it over to David Lynch, asking him to make it seem normal.
Seriously, there were times I honestly harboured the vain hope that Twilight was a feminist horror book, and I was so naïve I thought, "Wow, it can't get worse." But haha, yeah...
Moving on. Fifty Shades of Mr Darcy is for pun lovers. I love puns - I'm lame like that, I admit it. And despite my deliberate fall from the grammar nazi throne once faced with linguistic prescriptivism, I cannot let some things go when they are published by "serious" authors or as close as you can get to it. Fifty Shades of Mr Darcy does a wonderful parody of the horrid writing of Fifty Shades of Grey. I guess it served as a sort of balm - having read Fifty Shades of Grey and thinking nothing could make up for it, this book tells us, "Well, darling, you might as well laugh." And laugh I did.