I tried going all on GV a while ago on my iPhone 3GS with the people I tend to communicate with the most. GV integrates seamlessly with Android, but the iPhone integration left much to be desired. The app was really buggy and crashed on me all the time (~ 2 out of 3 times at startup) and all GV calls/sms had to go through the app. Another problem I had was that sometimes I would receive texts through GV and the notification would pop-up, but the app would never be updated for missed calls or unread texts so I would just completely miss the call/text. The app has probably improved by now, but I haven't checked.
I had a work BlackBerry at the same time and I actually found GV integration with BB to be much better. I could actually call/sms people with GV from my contacts list (though texting just redirected me to the app), but the number that appeared for people on the phone and in the Blackberry call history was still the GV assigned number as it is on the iPhone. GV texts also could not push to the BB app (they can push to the iPhone app), but you can convert GV texts to plaintext emails pushed to your BB.
On the other hand, among all the Android users I know, they've found the GV integration to be completely seamless. Overall, I think switching to all GV is great for Android users, but not so great for everyone else except under very special conditions (e.g. I had a coworker who had a work BB with unlimited data and voice, but no texting. She ended her personal cell phone plan and just used GV on the BB.)