I wouldn't mind GMail so much so long as adds are only shown based on the content of a specific email, its the idea of building up a profile over time that I'm a bit concerned about. With google going public you can no longer be sure what company policy will become if the share price starts to fall, they might start selling out the profiles to the highest bidder.
The safest thing would be for Google not to try to generate profiles, certainly not tied to seach queries, but just judge each mail individually

Add to that the fact that the Government can now request all the information a business stores about you without even having to show due process. You may think that "I've never done anything wrong, so why should I have anything to fear?", but can you be sure that you've never sent or received a mail that might be seen as a voice of dissent by some future more authoritarian Government?