"Service Battery" warning briefly appeared on brand new rMBP. Should I take it back?
I have a 4 year old Macbook and it occasionally shows the "Service Battery" warning. It doesn't appear all the time. It certainly isn't normal to have that show up so soon. I would take it back especially since you are very much within the 14 day return period. Even if they don't want to fix it, you could exchange it.

It sounds like you simply have a defective battery. Retina Macbook Pro's do not have user-serviceable batteries, which makes it even more imperative to get it fixed while you can.

Course, if you have AppleCare+, and were willing to wait and see if the battery life is less-then-desirable, you could wait. I'm not sure what benefit that would have, though.