A free game that is VERY addictive, but that's not necessarily a bad thing. There's plenty to do including trading, battling, questing, collecting, etc. You can pay to get the upperhand over others, but it's definitely not needed. You basically go through quests collecting cards and evolving them into stronger cards. You then choose 5 cards to include in your "deck" which you use to attack or defend against other players.
Some hints: At the beginning, you can put most of your points into stamina so you can do more quests. Attack and defense aren't really used until the higher levels when you start battling a lot and participating in events. Also, if you get a high rare, Srare, or SSrare, don't just trade it away quickly to the best offer. Try to find out what it is worth or you might get scammed. Some cards are worth significantly less because they cannot be evolved.
Here is my referral code:agt46884
You can put it in at the end of the tutorial if you wish. We both get a rare card and free rupies (in-game currency) if you do.
Hope you have as much fun as I do with this game! It works great on my HTC Thunderbolt. A few force closes every once in a while, but nothing too bad.
Get it from the Google Play store here: