It's transparent but definitely not user friendly. When you opt out it uses a cookie so if your cookies get deleted you need to opt out again. Also, since opting out uses a cookie you must opt out on every browser on every machine you use. Definitely not convenient.
With what account? Google serves on a ton of third party sites (and not just through AdSense, DoubleClick is huge). And that would require you to sign in to Google before you are opted-out, regardless if you were using Google or not. I use an plugin provided by Google to do the same thing, no worries about a cookie disappearing.