I sync my calendar from Outlook on a PC and everything else from a Mac. For Music, I haven't tried this as Music syncing is one way. If you sync with a Mac, then go to a PC, all music on your iPhone will be erased and replaced with what is on your PC. That's a pain, so I only sync music with one computer. It also took me a while to add all the album art to my music. iTunes, for some stupid reason, will only keep album art in it's library files and does not push it to the id3 tags on your mp3's. This means that if you copy your music directory from Mac to PC, all album art will be gone. This wouldn't be that big of a deal since iTunes can find album art for most music, but it couldn't find it for about 1/4 of my music, so I had to do that by hand.