Installed Textnow for the iPad. It's been working great so far. I initially tried textfree but they don't have an iPad app.
Textnow definitely works better though. They have mms support, you get an actual phone number, and they support Google Voice.
It integrates with your address book and contact pictures so you don't have to remember your friends numbers. Lastly it's FREE so you have nothing to lose!
If you want to download it their website is below as well as the iTunes link.