I have seen NNotify at one of my customers recently and tried it myself. Basically it gives you a better information about the simple fact, that you have a new mail.
The "Problem" with Lotus Notes is that the information about new mail is either a sound, a symbol in the Windows system tray or a popup window - or any combination of the three. Personally I *hate* the popup notification.
I hate to find that the text I was writing did not go into a document as I was writing into the new mail dialog.
I hate to try to work in the Admin client or Domino Designer and the new mail notification is constantly switching to the Notes client on every new mail.
The benefit of NNotify, compared to Notes Minder is that the flashing alert in the system tray is more visible than just a static "there is new mail"-icon.

What do you think?