Easy to lose but also easy to deactivate. This device can have up to 8 cards actively loaded at a time. Google Wallet can have several (no idea on how many). My leather wallet can carry about 15 cards of all types (ID, CC etc) before it gets too fat to be comfortable. Now what happens when these carry systems are lost?
Coin: can I just go to a single website and cancel all access to the cards, will that work without an active data conenction to the device, is it even necessary (with the phone connection system probably not depending on how frequent you set the check-in,) otherwise is there a pin I need to enter into the device before each use? If I do have the phone contact frequency set to a longer term, is their anyway short of digging out the original cards to get the 800#'s to call and cancel the cards to prevent their use? How long does a charge last on it, I keep multiple charge options avail to keep my phone topped off will I have to make sure I plug it in every night as well?
Google Wallet: The would be thief would have to guess my PIN to make any purchases, and all I need to do to deactivate fully is get web access anywhere I can (a friends phone, work, home etc.) and log in to my Google wallet account. Deactivate the service and it's totally deactivated until I get a new device and reactivate. Yes I'm out my phone but however that loss occurred it still happened and is not really relevant to the security of my payment systems. Any object can be stolen by a pickpocket.
Old Leather Wallet (aka the old style easy to lose basket all our eggs are currently kept in): It gets lost or stolen, lets assume I have 14 cc's and one DL in the wallet, I now have to find the numbers for and call to cancel every one of those cards individually, until I do they can be used online or for purchases under $25 with near impunity. If I act within 2 days my liability is limited to $50 or even less, But the money still gets spent and any losses above that $50 may not come directly out of my pockets but they come out of our pockets as the finance companies have to charge higher interest rates to cover those losses (only slightly higher but the losses to have to be accounted for).
Coin and GWallet are both far better than Old Leather Wallet. I'll go with GWallet as it's one less device, although the prevalence of mag readers to NFC readers currently makes that a weaker choice, but NFC readers are appearing in more and more POS terminals and so that drawback is fading faster and faster.
Coin looks good, but adds yet one more fragile electronic device to keep charged and to get broken or lost. Whereas GWallet or Isis combines that same capability with the device I already make every effort to keep charged, and keep with me. Yes loss of phone due to loss, theft or just a dead battery is a risk, but so is losing your wallet so as an argument it's weak at best.