Landscape typing for messaging is a MUST in your next update, I hope you release it ASAP.
Some small suggestions:
- Add some emoticons on the message
- Make clicking on the Viber Call icon won't directly going to call the person, I made many missed call because of this, should have make 1 more confirmation screen or message before calling the person >.<
- +1 to previous suggestion, an option to change ringtone and message tone.

After some time, even Viber service is running at the background, I still can't receive call unless I open up the Viber app for once. Happens to my sister's Galaxy Ace, and my Galaxy S2.