Back in High School this series was recommended to me. Someone loaned me the first book and the second was just out in hardback. I must admit the first was really good, but it was a tale all about future expectation. You are expecting the story to really take off for a few more grand books with a big finish. But as I read 2, 3, then 4, and with a last whimper 5, I had to admit all of that promise was failing miserably. So, as others were just entering the world I gave them fair warning about "how will it end?" complications with the series. My position was I would read all the books once the author managed to publish them all. Now I doubt I will even do that given the testimony of others on the quality of the current books.
One of my friends in college said his dad read the entire decology (the author's own term) or 10-book series by L. Ron Hubbard. After 10 books each 800-1000 pages long he said he learned one thing: GARBAGE IN, GARBAGE OUT. Thankfully I read Dianetics before reading the first 2-300 pages of book one of that series. The proselytizing was so thin, I almost believed the books were a mockumentary. But then why waste 10,000 pages of your life writing them? IMO, there is a serious mental condition involved there. And mixing with it with pseudo-psychological, pseudo-intellectual religion is a lowpoint in the history of fiction writing.
I guess my advice for reading unfinished book series is .... look before you leap.