I've been using Chomp since I moved from iOS to Android. It's got a really clean interface that seems reminiscent of iMessage, but can also be customized. Very easy and efficient. Plus, the app icon is cute!
I recently switched to using Google hangouts, and one of my favorite aspects to this is it works across all my devices (2 cell phones, a couple tablets, PC, laptop, etc.) very easily and my SMS messages are equally and immediately available no matter what device I'm using, which can be very helpful. I no longer have to tell my friends WHICH of my 2 cell numbers to text, or to send me an e-mail instead because I happen to be using my PC.