I'm a little fond of All Our Yesterdays.
Terrill writes the book in a very weird format – it takes awhile to get into the story and get a grip of what is really going on. Marina is a self-conscious person who lets her friends dictate everything for her – how to win boys, how to dress, how to talk, etc. Marina just comes across as a very shallow person hoping to win over the love of her best friend, James Shaw, while trying to find out who is attempting to murder him.
Em, on the other hand, is someone completely different – she's more determined, went through more trauma... Basically, Em has been through more than Marina, and I think she's a vital asset to the story's enjoyment (Marina plays a vital role as well, but if it were just her, it would have been boring). She teams up with Finn in the hopes to shut down Cassandra, a time travel device created with the intention for good things (stopping wars and disasters, for instance) but later became more of a problem rather than for everyone's good.
But back to the whole weird format. Since I've never actually come across Terrill's format ever in another time travel book, it's completely mind-boggling. One minute it seems like both Em and Marina are the same, the next, they're completely different. The only constant variable going on throughout the entire book is James and Finn (even those two were different and the same – they were just obvious). It really just takes awhile to realize the time period is the same, but the viewpoints are different.
Quite literally, 350+ pages of All Our Yesterdays is dedicated to getting rid of the evil mastermind behind Cassandra, but it's so much fun seeing how Terrill clicked the weird format so well together.
P.S. I personally think All Our Yesterdays works out just fine as a stand-alone. Although I would love to see a sequel and how Terrill will take the story now that the main problem has been solved, I don't really see anything that could happen aside from a "tragic" love story.