I guess there's a couple things I want to clarify. I don't think reader reviews are unreliable at all; ultimately, it's the readers' opinions that are the only true test of the quality of a book. A good book is one that people like (or one that YOU like, I should say).
My problem is that I don't know whether a reader review is really a reader review or not. It could be a friend/family/author review, in which case it's really an ad disguised as a review. I suspect that's very often the case with self-published books on Amazon, so it's led me to mistrust a lot of the reader reviews for self-published books - which is unfortunate.
Is it possible to find a good book on Smashwords by reading through the sample for each book? Well, it depends on how much time you have. At the moment, finding a good book on Smashwords is a remarkably time-consuming experience. So much so, in fact, that at the end of the day I'm more likely to do something else in my spare time, like posting on a Mobile Reads forum

(which at least gives me the chance to have my two cents' worth).
None of this is intended as a criticism of Smashwords authors. It would be a little inane of me to condemn Smashwords authors because I AM a Smashwords author, ironically enough

. There are unquestionably good Smashwords books out there. Here's the question: how do you find them without spending six years on your search?
I think (as I said before) that all of this is a temporary problem. The Internet is still young and online self-publishing is a relatively new trend. I think a kind of crowdsourcing model is going to emerge that will solve some of these problems. In the meantime, however, I commend those who are reviewing Smashwords or Kindle books to make it a little easier for the rest of us.
And yes, I still think a book-reviewing AI would be a really big help. If any of you are computer scientists, this is it! here's your chance to help us all out!