I rarely use shorteners, so excuse my ignorance here, but what does Google offer here that bit.ly doesn't? Why would I switch to Google after using bit.ly for a long time?
One nice thing is that goo.gl is pretty fast. See the link at http://techcrunch.com/2010/03/17/url-shorteners-speed/ for a graph from a few months ago, for example. We're also shooting for great uptime and we have a bunch of malware checks when shortening urls.