I'm not sure if it should be a 1* or 2* for me. Let's say 2, for the Terminator shout-outs, which made me smile (I watched T1 and T2 when I was in middle school, and I still have fond memories of those), for the couple of good things I liked, and because I didn't actually want to throw the book through the window. In fact, I think it could be nice for a younger audience—maybe 7th graders—because if you don't pay attention to the plot holes, well, the story has the potential to be a fast, entertaining read (though a bit frightening and gruesome in parts for the really young readers out there).
The characters weren't particularly well-developed, but neither were they insufferable, and I appreciated the plot not being bogged down by the useless romance I see rearing its head in too many YA novels. However, the plot holes are what sunk this book for me. It's dealing with time travel, a very, very tricky subject, and one that is really not so easy to master. As soon as it enters the game, it brings its lot of questions: what's the science behind it, what about paradox, what happens if a character meets him/herself from the past, and so on. Unfortunately, TimeRiders didn't deal well with that in my eyes.
Here's an example: the characters live in a sort of "bubble", from which they observe the same two days in time. Within the bubble, they age normally, but every couple of days, the world around them is reset, and reverts back to what it was at the beginning of their observation period. One of the characters' role is to stay outside, keeping an eye for whatever may be different, a sure sign that a shift has occured somewhere in history, and report it to the others so that the team's analyst can locate the problem, and the actual timr-travellers can go there to fix it. However, there's no explanation as to why this character isn't affected by huge time shifts.
That kind of problem is never really addressed nor explained. I would've been content with something as simple as "once you're plucked out of time, you can't be affected by shifts anymore for [insert whatever reason]", but I don't even remember seeing that.
Another thing I wondered about was the whole time agency business. The teenagers are never introduced to it, except through what Foster tells them about it; no other team is ever seen or even mentioned; and I had the feeling that it didn't really exist, that those three kids and their old mentor were the only ones in the world. Maybe this will be explained in book 2 or 3, I don't know; still, considering this is the book in which the characters are trained for their missions in time, it would've made sense to give us more information about that, to make us actually hear about other teams. (Again, I could've gone with a short explanation, maybe a rule such as "each time is assigned to a given time period and forbidden to talk to the others, for fear of time paradox." Whatever.)
Not the worst story I've read so far, but consider it a 1.5 on my scale, not more.