A check-in from the front lines:
I've been doing some cross-platform play (loading cities on either PC or OS X, depending on which OS I'm running) and *this* is why server-side saving is a must.
How many games (for any OS) have different save-game formats for different operating systems? (I don't know about you, but this has been, in fact, one of my biggest peeves about multi-platform software in general, let alone gaming - different file formats.) Server-side saves allows for save-format abstraction - it doesn't matter what OS the game client is running. (I have expanded my server count to a staggering nine - NA East 1-4, NA West 2-4, Test, and - don't laugh - Antarctica. The southernmost server for Simcity obvious has a reason for being - and it's NOT a test server; that is completely separate.) I flat out LOVE being able to pick up exactly where I left off - and have the OS I'm running not matter whatever.