"For someone to invade something, there must be mutual disagreement of this invasion taking place. If you sign up with Google Inc's Gmail, you fully agree you give up your privacy. You did so when you choose Hotmail, Yahoo, or any other free webmailer. This is not to say anybody can sell your privacy to third parties. And Google does not; all an advertiser will see is that his "wine" or "beer" or "coke" product campaign got N hits. The advertiser will neither know what is in your email, nor do they have any interest in that information. It's simply not commercial."Couldn't have said it better myself, emphasis mine.
"Now would you be creeped out too by Gmail? Think about it. Ads are all around you in free email services. But now one thing, only one thing changed; those ads will become relevant. This means they change from being noise, to being something that might actually help you. I often find the AdSense ads I display on my sites add to the overall information, instead of subtracting from it, or annoying my visitors. And if all fails, you still have the chance to ignore the ads presented to you. No one is forcing you to click on them."
Update: The first Gmail review: "In fact, the system is so straightforward that there isn't really a great deal to comment on. The adverts are very similar to their usual Google search counterparts, but they don't get displayed all the time... At least, unlike certain providers, Google won't automatically spam you as soon as you've signed up."