The biggest problem,is that you can't use it together whit iCal. Events made in Sunbird, wil be deletet - when iCal updates it's calendar...
But as a standalone app - it works fine on WIN2000 and Mac 10.3.
Just like Firefox and Thunderbird, it needs time to grow ;-)
Mozilla Sunbird
Use a XUL-based calendar app.
Version: 0.9