As the other thread on here says there are just to many free ebooks to choose from and a good percentage of them are crap. They problem with ebooks is anyone can publish one and upload it to a site like Smashwords or Feedbooks. To get a book published requires that it be good enough to make the publisher believe it will sell, with free books there is no middleman between us and the writer.
I think most people go through a phase of downloading pretty much any free ebook they can find and end up with hundreds of unread books on their computer/reader. I know I certainly when through this stage and am only now coming out the other side. I have read some absolute rubbish to the point that I have actually deleted a few books after only reading ten pages because if they haven't managed to grab my interest by them it's not a good sign (
Don't Mess With Earth being one I felt was so bad I had to write a review).
Occasionally you will find a gem of a free book (I am currently reading the Northworld Trilogy which is a good read) or a good author (Cory Doctorow is one such author in my opinion) but a lot of the time you will find work that has not been proofread and has not had anybody helping the author by reading what they have done and pointing out where improvements could be made before publishing.