I have now tried this out for a while--not having my gmail browser tab open all the time, switching off google-notifier, and only checking my email about 3 times a day (a bit more frequently when I actually have to write an email as part of doing a job).
I feel I am much more productive, get things done, and go home with all items on my daily to-do list crossed off (which is a great feeling!). And I have now just come across a piece on the BBC website where researchers say that constant email alerts disrupt disproportionally.
So far I don't miss it much. Occasionally I get a twinge, but can easily resist the urge to check. I'm always a bit shocked when I've got 10 new emails, as before they didn't mount up because I dealt with them as soon as they came up. Guess I have to get used to that...
Needless to say, I have also cut down on twitter and google reader, both of which I only check when I have time, eg when all my daily tasks are ticked off. Productivity Heaven!