Let me start with a disclaimer: this post is not against the tool, but against what I see as a toxic habit.
This NetBeans plugin enables notification in the IDE when new Gmail messages become available. I am sure many people will find it very useful, yet I am trying to draw my personal battle line. We are overwhelmed by distractions, and adding another one would only worsen the situation.
Do I really need to check that e-mail immediately, or can I safely ignore it for another 15 minutes? If you use the pomodoro technique you organize your work in short 25 minutes stints, so the average delay is absolutely trifling. The same holds true if you use similar time management techniques.
Interruptions break your flow, and getting back to what you were doing becomes increasingly hard. That's why you should strive to manage interruptions, not to be ruled by them. A notifier is an interruption, and as such should be dealt with.
I normally check my e-mail(s) between pomodoros, and that's not a secret for anyone: I am not at the service desk, so my "flow" periods are not 5 minutes bursts: there are days in which I don't even check the trouble ticketing system. I admit this could sound excessive, but there are many other brilliant people on the responsibility chain before me, and this could loosen the stiffness in your back a little. I am confident that if a pressing situation arises I'll be reached by a telephone call, and if the situation requires it I'll manage the interruption catching it and trying to solve the problem as soon as possible. I'd like to say "as soon as it is needed", but you might have noticed that saying no is not so easy. It is even less so when we're saying no to ourselves.
Yet, if we want to put the most precious resource we have to good use, we have to learn how to do it.