Google Blogoscoped quotes a German podcast that asks "why do we trust Google more than Microsoft?". I found some diggers' comments more interesting than the podcast, so I will quote two great opinions:
YES, I TRUST GOOGLE
"People who think Google does anything with your e-mail that other webmail services don't do are the ones who don't have the facts. Gmail basically scans your e-mail for keywords so it knows what ads to fetch. The mechanics of it aren't really any different from a spam filter or spell checker, and the likelihood of abuse is absolutely no different. In fact, this wouldn't even be the most efficient way to abuse your privacy. If they wanted to do something nasty with your e-mails, it would be done behind the scenes without necessarily having any connection with an ad system or anything else. And in that respect, any public webmail system on the planet could be doing whatever they want with your e-mails and you would have no way of knowing. Yes, you have to trust that Google isn't abusing your rights, but it's no different than any other public webmail service.
Again, those who think Google is doing anything more privacy-invading than other search engines or webmail services are the ones who don't seem to have the facts here. From a technical point of view, all such services have as much ability to follow your behavior and report it to the government."
NO, I DON'T TRUST GOOGLE
"I don't trust Google. The amount of tracking they do on everyone is worrying. It's not Google per se that worries me, as they have a pretty decent track record, but it's who will get access to this information in the end. What if Google is bought by another company some time in the future? Or if laws are passed that allow various governments access to our data?
Google knows pretty much everything about us, from our search queries, our surfing habits tracked by Google ads, they know where we live and where we go by our Google Maps queries and if you have a GMail account, they know a lot more about you personally. It's a gold mine."