Most of what you read on the internet about Google Voice is simply wrong, misinformed, outdated, or just plain dumb.
I've already described the background behind the May 15th changes, what will change, and what won't change, in my post here:
http://www.obitalk.com/forum/index.php?topic=7750.msg50190#msg50190Aside from that, the single most misunderstood thing about the mobile Google Voice app for Android or iOS, is that is not, and never has been, a VoIP (internet voice calling) app. It cannot make or receive calls over WiFi/3G/4G. It only manages calls over a working mobile phone's active voice network connection.
After May 15th, the mobile Google Voice apps will continue to be able to make and receive voice calls over the voice network carrier's connection, and they will continue to be able to send and receive SMS texts over the internet (so, those are two completely different functions, provided by two different gateways).
Today, the Google Hangouts app for Apple iOS devices can make and receive voice calls over the internet (VoIP). At some TBD (hopefully soon) future date, the Android Hangouts app will also be able to make and receive voice calls over VoIP, but it doesn't have that capability today.
Finally, at some future TBD date, the Voice and Hangouts, confusingly different, and somewhat overlapping user interfaces (the apps themselves) will merge into one app. The underlying services provided are expected to stay the same or improve. In other words, the Google Voice service isn't going away, but the way you use it on a device or web page will look different, as it is integrated with Hangouts.