"Turns out, in its current state, the card will only use your remaining Google Wallet balance when making purchases. This means if your balance is $0.00, the card will come back as declined. This is very much different from the app, which will use the primary credit card you have set up on Google Wallet once Google Wallet funds have been exhausted. But we still have hope… According to Googler Jeff Craig — who’s been using the Google Wallet Card for quite sometime now (working at Google has its perks) — his version of the card acts just like the app, using the remaining Google Wallet balance before switching over to the primary card on file. This is much how the PayPal debit card works (after adding your bank account), and it’s odd to not see this “feature” in the Google Wallet Card.
It’s entirely possible Google will change the way the Wallet Card works in the future, but until then, it’s nothing more than a pre-paid credit (read: kinda lame). We’ll let you know if we hear more."
There are a lot of security concerns, and even legal concerns with Coin coming out now. Not to mention it's $100, and after 2 years the battery dies and you have to throw it away. I'll go with the free and secure Wallet Card.