Used to be pretty bad when it was .mac but much better now.
I have an iPhone and Mac at home and use it to sync calendar and diary using push notifications. Don't use the email much but the 20gb storage is useful for sharing files. Uploading to it via the desktop is a bit painful so I use Transmit FTP client instead. Shared files up to around 4.5gb on it.
Email is actually quite good and you can also create multiple aliases.
All the features can be got elsewhere but I find it's least hassle to have it all in one place.
Oh and if I had another Mac I'd sync the settings as above.
If you have an iPhone the remote kill switch is pretty good, as is the locate my iPhone.