I did enjoy reading this unusual and clever book!
Unusual, because I have been reading sci-fi for a very long time, and particularly like it when I see a new idea, or new spin come up.
Clever, as it was that interesting spin on portals/gates between worlds, and especially the way the worlds were connected - difficult not to say too much here - which was well thought out, and easy to understand, yet had that element of physics that I did enjoy. But not too much, so don't get put off by my comment!
The characters were good, and believable, with some being rather nicely crafted. Lots of action! The story was one that I found very difficult to put down, and was finished all too soon.
There is a nice complete ending, but I would just add that the book only really makes sense if you have read the first one in this series.