I know the POV changes from book to book have been a subject of debate. I honestly had no problems with it from Hourglass to Timepiece. I loved Kaleb's persona in the first book, his cocky attitude but sweet nature, and it carried over wonderfully the second time around. I completely understand why the decision was made to switch it up once again, but I must say I couldn't help but feel the loss of my favorites from before.
Dune is someone we've already met. Though I honestly do not remember him (more on that later). He was very sweet and almost quiet with a sad past and an interesting ability to control the tides. He travels to New Orleans to continue his research on the infinityglass, which turns out to be a person, and our other POV, Hallie. Hallie makes up for his quietness. She's confident and knows how to get what she wants, which may be partly due to being the daughter of a mobster. She has an extremely cool talent of mutating her appearance, a la Mystique from X-Men. Her father often sends her on dangerous missions involving theft due to that very ability but was extremely protective of her as well. That whole scenario confuzzled me, to be honest, but it led him to hire Dune to continue his research under the disguise of a new bodyguard for Hallie. These two were cute together, especially when Hallie would try to get a reaction out of Dune. I didn't quite fall for them as a couple as much as with the previous books. It felt a little rushed, but I still enjoyed watching them fall in love.
As for the actual progression and conclusion of the storyline throughout the series, it seemed to have wrapped up nice, though I can't help but wish the climax was a bit longer with a little more 'oomph' for a last book. The rips were pretty creepy this time around with our Hallie. Her powers are crazy strong and I almost wish we had more time to explore them, same with Dune. The epilogue though... brought crazy smiles to my face!
This is where I express my absolute wish that I re-read the first two books before the conclusion. I forgot A LOT of details on characters and what was actually going on with the rips and the 'bad guys'. For instance, I didn't remember Dune. I barely remembered Poe and Teague. Sure, pieces came back and the author was able to put a few details in to help remind me, but it wasn't enough. And I think that is something you face with switching POVs around with each book. The new characters are going to relate to and have a different past and experience with the other characters. They will see them in a new way so it's harder to place them after a whole year in between books. I'm not sure if it is just this last book or if it was just something I didn't notice before because I read Hourglass and Timepiece back-to-back and had no problems. All I'm saying is, I highly recommend having the details and characters fresh in your mind before undertaking this last installment.
Having said all of that, this is one series I recommend! This is where my love of time travel novels stemmed from and I thank the author for this journey she took us through and the amazing characters she brought our way.