If it was just a matter of choosing a fitting number of stars, I think this would be closer to three than two. As it is, I'm going by what Goodreads says the stars mean, and this was more an "it was ok" book than a "I liked it" one.
Not to say it was a bad book - no, certainly not. As a middle grade / young adult (not entirely sure which category it fits) time-travel action-adventure, it was fine, with plenty of exciting bits.
On the other hand I didn't connect with any of the characters, which is never a good thing, didn't much care whether they died heroically or lived to save the day (and the world), and I just couldn't suspend disbelief enough to buy into the whole "we're the only people keeping history on the right track, and yes, "we" means three untrained teenagers who go through four days of living in a time bubble and one lousy training mission which actually didn't prepare us at all". Oh, and of course
a kid who's just been saved from going down with the Titanic is going to go from "what are computers? are these cars?" to effortless time-travel and fitting in just fine in five days.
So, meh. Nice concept, decent excitement, but all in all, a bit disappointing.