It's brilliant, probably one of the best apps out there. It's not just about 'free texts' its much more than that. Whatapp is not texting, it's instant messaging really, but you can send images, video, audio and smilies too. You can see when the other person is typing and t turns it into a 'conversation' and not just texts back and forward. Also, its not like skype etc where you have to know the username of your friends who have it etc, it automatically knows from your phone book who out of your existing friends uses it - so they are all already in your address book when you start. Give it a go, its a must have app.
It's free for android users, usually it costs for iphone users but they have made it free fo the holidays - so get it now before it ends!