Paul Thurrott does a nice job of breaking down how calendars work on Windows Phone 7, specifically the trade off between simplicity and ability.
Windows Phone 7, via Live and/or Exchange, can sync up your calendar events. You can also add external sources like Google to the mix. All of these can by synced to the phone is a very easy fashion, the problem though is syncing back as you can only have one "source" that gets synced too.
Then there is the issue of multiple, nested calendars that are "subscribed" too within a calendar e.g. 'weather' within Google calendar--those don't get synced.
Granted, this is v1.0 of calendar support in WP7, but obviously some "power users" will have a tough time swallowing those limitations, while for us with not much to do in our lives (ahem, raises hand) it won't be a big deal. As Thurrott notes:
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Daniel Rubino is the Editor-in-chief of Windows Central, head reviewer, podcast co-host, and analyst. He has been covering Microsoft since 2007 when this site was called WMExperts (and later Windows Phone Central). His interests include Windows, laptops, next-gen computing, and for some reason, watches. Before all this tech stuff, he worked on a Ph.D. in linguistics, watched people sleep (for medical purposes!), and ran the projectors at movie theaters because it was fun.