To review, or not to review

I don't want to quash someone's creativity, but I accepted a book to potentially review it (in order to post the review to this site, I have to have honestly rated the book at least worthy of three stars) and I can't.  I am over thirty percent of the way through and it is a mess.  I couldn't even give it two stars.  It desperately needs an editor- grammar-wise and plot-wise.  The idea is intriguing and with some extra work, there is potential here, but as is?  No.  


I'm too nice, I guess, but I have to either decide if I just want to ignore this author, and move on to the next book hoping they don't contact me, (I have a pile already from this site alone- never mind Edelweiss and NetGalley) or I send the author something to the effect of 'thanks, but this wasn't my cup of tea and I can't honestly give this three stars'.  


Then what do I do?  Do I review it anyway (rules of the site say I can do that if I have a blog)?  Or not?  Granted the book is only being sold for about .99 cents, but if I spent even that little on this book, I have to be honest here, I'd be pissed.  I do plan on finishing the book, but from what I have seen thus far, I don't think it is going to get any better.


Where do I draw the line between giving my honest opinion to other potential buyers or simply saying nothing if I don't have any rainbows of praise to sprinkle liberally over the tome?