Another of those "somewhere between three and four Goodreads stars" books for me. I did enjoy it, quite a bit - a fast, fun read, with lots to like about it; the first half of it would probably have been a solid four. Uncle Percy was a likeable character and the sabre-tooth tiger was a nice touch.
What let it down for me, ultimately, was Becky: I didn't feel her character was consistent and, well, spoiled modern teenager or not, I just had a hard time believing her sudden outbursts at times (yes, I know, teenagers throw tantrums, but at times when it's extremely likely to get them killed? hmm, well, yeah, they still might, if they're exceptionally stupid, I suppose). And I also didn't care for the ALL CAPS DIALOGUE whenever anyone shouted or yelled something. Yes, we know they said it in a loud voice; that's what the shouting and yelling in the dialogue tag indicates - having half a Kindle page in all caps every now and then became tiresome quite quickly.
That said, I think that if I were ~25-28 years younger, I'd have loved this book a great deal. I'll probably also pick up the sequel at some point - it was certainly enjoyable enough.