I have it and it seems to work fine with my Droid Incredible. But I haven't actually used it with any clients yet...
One obvious feature that is missing from Square, is a Visa/MC debit card that is tied to the account. If I accept a payment with Square, they will initiate a transfer to my bank account "within 24 hours", but I might not actually see anything for up to 3 days. Sometimes that delay can be unacceptable.
With Paypal, once my invoice is paid the funds are available immediately in my Paypal account and can be accessed with the Paypal debit card. There is no need to wait for a bank transfer to take place. :thumbsup: