I think Google Apps compares pretty well to just about anything else out there and the price for the standard version, free, is obviously great. Keep in mind though, that if you want do manipulate calendar data from your own domain, you will need to subscribe to premium.
Data API Developer’s Guide: JavaScript
If you can do without that kind of integration, standard should do just fine But here’s just one example where premium is required.
Say, you have a calandar application in use on your website with extra fields that don’t exist in Google Calendar. Before you can start thinking about combining the two, you will need a premium subscription.
