Because your app is just that, an app and not a deck of cards, I moved the topic to the Conversation Parlor.
While you do have the convenience of not having to use play-and-pass, you do have the drawback of requiring Internet access. Sure, most phones have it, but in major cities with underground subways, basements, enclosed shopping malls, etc., anywhere the cellular signal fails and WiFi isn't available, it's "game over."
Making it free is good and give it an edge over the purchase of a cheap pack of cards. Does it have a chat feature? Since play occurs over the Internet, you can have people playing literally from other countries against each other and they may want to use the app for remaining in touch.
How does it handle a lost connection? In the US, there's two common cellphone protocols in use, GSM (popular in Europe) and CDMA (popular in parts of Asia and South America). GSM is fine, you can have a data connection running at the same time as a phone call. CDMA, however, is a different story, and two of the top four phone networks in the US use CDMA. In CDMA networks, a phone call will disconnect a smartphone from the Internet for the duration of the phone call. It's a hassle for me when I'm using my phone for navigation while I'm driving, then suddenly receive a call and have to either ignore it until I reach a known location I can "self-navigate" or I answer the call and don't know where I'm going.
There's also a few legal issues you'd want to clarify, at least for your customers if not for yourself. In the US, some states are getting more and more restrictive about use of cellphones while driving. In the state of New Jersey, for example, it's now illegal not only to text while driving but even to RECEIVE a text while driving. How would NJ state law cover a request from one of your players trying to start a card game with someone in their state while that person was driving? Would the receipt of the game request constitute a violation of state law for the recipient? Would you need to include a disclaimer of some kind to prevent or at least warn your players about the risk of violating such laws?