DOUG S.  While I prefer this to other solutions I've seen in some ways I still use alternatives if only because (1) they support multiple accounts—I have four—and (2) this has no support for Growl, something I find extremely surprising.  
(Version 1.10.4.676)

2SHAE  I like this app, but recently my Google Notifier quits by it self. It just disappears from the menubar.

Also, it's to bad that it doesn't check every minute anymore. Sometime it takes 10 minutes before it checks...kinda ridiculous.  
(Version 1.10.4.676)

SHARPIEEE  as has been mentioned by others, this really needs a few more options in its preferences window. i'm kind of confused why google didn't provide more choices. i personally don't need my notifier to check every minute, or every 5 minutes for that matter, and yet there's no apparent way to choose the time between checks. i also had frequent problems with the notifier glitching out, weird problems with the email checks failing and never starting back up, etc. its a great utility to have, i just hope google puts a little more polish onto this.  
(Version 1.10.4.676)

RORO01  BEWARE!!!

This version checks for mail every 5 minutes as opposed to every minute in the previous version. It would have been better if the developer had given us the option in Preferences to choose the interval like in Apple Mail.  
(Version 1.10.4.676)

MACUPDATE ADMIN  The download link here at MacUpdate points directly to the Google server, you are downloading Google Notifier from Google and there is no Google Updater app installed, only Google Notifier.  
(Version 1.10.2.580)

FREEBSD  Google Notifier downloaded directly from Google will avoid the Google Updater junk that you can read about in several other reviews.

That aside, once you actually get the Google Notifier.app installed, you may encounter the same problems that I have seen with respect to latency, redundancy, and time outs.

I assume that Notifier checks for new mail each minute, which is a fixed time that has no easy preference change that I can see. However, this operation times out frequently (several times per workday), irrespective of the state of my internet connection, which is fine and always on (and working during these time outs).

Additionally, Google Notifier will from time to time ask for my login credentials again, to erratic behavior. When it asks for this info, pressing cancel seems to have the same effect as pressing OK. Yet the annoying pop up dialog box invariably disrupts my work once or twice a day.

Finally, I have noticed that, though it looks to have been checking successfully for new mail within each minute and finding nothing new, checking Gmail manually in a browser has sometimes yielded new mail that is greater than 1 minute old. It looks like Google Notifier just missed this mail. It's unclear whether or not Notifier would have eventually found it, as manual checking from Notifier did not find the new mail. Strange.

I do find this app better than the other ones I've used, for simplicity and size. However it's imperfect and could use a more rigorous testing and release schedule to be truly useful.   
(Version 1.10.0.522)

JQMUSICMAN  Mail notifier doesn't work with Google Apps hosted mail. Gives no error at login but never alerts on new messages. Totally lame.  
(Version 1.10.0.522)

LARRYMCJ  Using this with Camino and the green notification icon with the exclamation point just stays on top of the Menu Bar Notifier icon.   
(Version 1.9.97)

ELEVATORIUM  I'm pleased this works with Google Apps for Your Domain. Good stuff, and best of all, it's free! :>  
(Version 1.9.97)

SWGS1  I use and enjoy the Google Notifier software, it works as advertised, and I'm glad to see Google doing more Mac software. But, the older version was able to use Growl notifications, which I prefered over the homegrown solution that is now included in Google Notifier.

Until google starts supporting Growl notifications instead of their own system, then I'll just use this without the visual notification, and just have it in my menubar. I know this is a minor thing to gripe about, but it matters to me, I don't want two different kinds of non-standard screen notifications.

For those that aren't familair with Growl, go to http://growl.info  
(Version 1.9.90)