Wow, I didn't even realize Topps was planning to do this. I play their MLB (Bunt) and NFL (Huddle) versions.
It's very similar to Upper Deck's epack, except that it also has a game aspect. Instead of getting the free base pack like Upper Deck gives you, you get a certain number of coins each day. And of course, you can pay a lot more real money if you want a lot more coins to use. It depends on how often they keep up with it, but in addition to a base set, they'll have special inserts and contest prizes. In their other apps, they have "autographed" and "game used" ecards, but I don't really understand the appeal since there is no possible physical card to get.
There is also trading, but it can be pretty cutthroat.
Anyway, I'd encourage members here to try it before knocking it. I've played the other two without putting down a dime, and I get enough enjoyment out of them that I've now downloaded Skate as well.