This is exactly what I've been needing.
Google Notifier only checks for new messages every 15-30 minutes, whenever it seems to feel like it, without an option in preferences to set how often you'd like it to check. Gmail Notifr had that option, but made itself pretty much useless by not showing you who sent your emails or the subjects of them.
Notify has the best of all worlds - you can set it to check for new messages at lots of different intervals; you can see the sender, the subject, and even the first couple of lines of the email itself so you can decide if it's worth opening Gmail right then to check it; and I think the tabbed window interface is pretty darn clever. And I'm sure it's handy for lots of people that it can check up to four Gmail accounts, though that's not something I use myself.
Notify, for me, is perfect as-is and just what I've been looking for. If they keep adding to it, great!