Call-backs: Unlike text messages, Google Voice doesn't provide a magic call-back number to avoid re-introducing your "old"/carrier number to contacts. If Voice's mobile or desktop web sites are accessible, you have to call into your Google Voice number, hit 2 to place a call, then enter the number you're calling back. That's a pain in the rear, and even more so if you don't feel comfortable setting your cell phone to not require a PIN when you call Google Voice.
If you're calling certain contacts all the time and want the calls to go through Google Voice, you could search to see how to insert one- or two-second pauses into a phone number in your phone's address book. Then create an "Other" number for them that consists of your Google Voice number, then a pause, then "2," then a pause, then that contact's actual phone number, followed by a "#". That automates the Google Voice dialing, leaving you with just a bit of a wait while it goes through.
How to Ease Your Transition to Google Voice via Lifehacker