Here are sixteen quotes (verbatim, typos and all) from recent Amazon reviews of The Penal Colony.
1. Unfortunately upon reaching the end I felt unsatisfied due to the author leaving numerous loose ends. This ruined the enjoyment for me.
2. (And with a satisfying ending, too)
3. the ending left a lot to be desired, very open ended and a bit of a disappointment.
4. I always like a happy ending.
5. Not a bad story, would've been quite a bit better if it had been edited better. Also the ending, abruptly one page. C'mon...
6. Great read and loved the ending.
7. The only downside I could find was that the ending of the book was a little too quick for me and I had to read the last chapter a couple of times to see if I had missed something hence I only scored it 4 stars.
8. Well done from beginning to end.
9. I wanted more information about the end the the trip. Without giving spoilers I wanted more of the end . How the trip went. What happened ? The author could have given more there and didn't . It was an ending without a real ending if you like that sort of thing
10. It had a good ending, it was smart, and it spoke to redemption.
11. I felt short changed at the end, but enjoyed it overall.
12. A wonderful story allowing for great character development throughout and the inkling of "I wonder how he fared" at the end.
13. This was an interesting read and the first half is great but by the end it was like, enough already.
14. it is a good story though and has a pretty good ending.
15. I was glad I stuck with it, but have to say that the ending was a bit of a let down!
16. I also want to go on the record and say can you teach other authors to write endings? Yours was damn near perfect and there were no silly cliffhangers or any such nonsense, just a beautiful ending. I just don't get to read but one a year.
See also here.