Having used Gmail Notifr for a few days, I thought I'd return and give it a more objective review (as promised) now that it's working for me.
First up, I'd like to give kudos to the developer, James. There was a problem with 0.3.0 in saving usernames, and he worked quickly and non-stop until he'd resolved the problem. Top marks!!
I haven't tested this for multiple users, I only have my single account to monitor, but it did that without any problems at all. The ability to adjust the polling of the Gmail server is a nice feature.
There are a few things missing that the Google Notifier does that Gmail Notifr doesn't. A "Compose Message" menu item would be useful. I think the message list could possibly be expanded on too. Google Notifier, for example, displays a tooltip with the beginning of the message in it. I find this very useful.
One or two people have commented on the memory usage of the 'official' google notifier, however, I've had both running side by side for a few days and Activity Monitor is telling me that Gmail Notifr is using a little more (19.52 MB) than the Google Notifier (16.87 MB).
Gmail Notifr does what it is meant to do, well, and with a very responsive, and nice, developer to boot. However, in my situation, I have decided to stick with the Google Notifier because I don't need multiple account checking and I also like to have the list of events from my Google Calendar. I use Busy Sync to synchronise my Google Calendar to iCal, but I'm yet to find a nice solution to listing events and emails nicely as the Google Notifier does.
I wish the developer all the best with this project, and I will certainly be keeping an eye on it.