I don't see the need for a 3rd party to interfere with the process. Google doesn't have the secure infrastructure setup like the credit card companies who have been doing it for eons. It may be that this is just an app that actually calls on the credit card network in the background, so Google isn't really providing anything but an interface on top of everything. I'm sure this is what's going on.
I would rather use a wallet app created by the credit card companies themselves. They are the masters in security, not Google.
I will be interested to see how they secure the transaction process. I have confidence that Google will figure something out, but then hackers will find an exploit and the the endless battle will continue.
They imply this will let you get rid of your wallet...yet it won't. Still need your ID and having a $20 bill on you at all times is still a good idea for emergencies. I'm also not sure if this is really all that more convenient.