I use Google Voice for texting. It's great. Notifications are reliable, there have been no delays in receiving messages (or others receiving mine), and of course you can access it from your computer as well. Super handy, and no SMS charge.
As for voice calls, I also used to use it for that. The app manages everything nicely for outgoing calls -- you can set it to make every call "from" (show up as) your Google Voice number. And you can set it up to forward all calls to it to your "real" number -- so no one needs to know it except Google and you can give everyone your GVoice number. Plus, every call to the U.S. and Canada only uses your local minutes (or charges you a local rate if you don't have a plan).
Note: You can't receive texts from services such as Facebook that actually email you via [yournumber]@[yourcarrier].com or something similar, you cannot text short codes such as FBOOK (32665), and you can't send MMS messages (but that's what email's for amirite?).