GV Mobile+ still has a lot of features that this official google app doesn't match, like the ability to paste phone numbers into the dial pad and ability to rout numbers via Skype.
GV Mobile+ has a lot of extra features, but I feel like the native Google Voice app is faster and aesthetically pleasing to the eye. There were also always glitches with the GV Mobile+ app I used, e.g. text disappearing or hiding behind a whole line of whitespace.
Now that they have an official app, maybe they'll iterate fast to catch up to GV Mobile's features.
On the other hand, if Apple really didn't want to allow the app, and only did so to satisfy regulators, they can do their best to slow and stall on every change that Google submits.