Characters are well established.
# of memorable endings: 1 mixed
The time travel path is crummy! There is only one interesting ending for that path (albeit a very good one), and the rest are depressing. I tend to like bad endings, but these bad endings were arbitrary and not due to some foolish action on your part. Yes, there is one way you can make it back to your own time, but none of the choices logically lead to that ending, which makes this storyline consist mainly of random wandering.
The storyline involving the puzzle is decently entertaining. The puzzle is not as good as the puzzles in Carol Gaskin's "Master of Mazes," but nonetheless still worth a read.
This book is very similar to "Spy for George Washington" in the respect that:
* one main path is interesting and the other is dull
* it is easy to get a good ending (for the interesting path)
Both books are entertaining as long as you choose the interesting path.