While we're waiting for the real Spy updates and unlockables (and not fakes which momentarily fooled even Valve) OMFGNinja.com has gone and created an amazingly deep three-part advanced tutorial on how to play the Spy. It is mesmerizing, and showcases how any character in this game, with enough skill and planning, can become downright unstoppable. The leaping backstabs in part one, above, are jaw-dropping. The other two videos are at OMFGNinja, so follow the link and give them some credit. Youtube resolution sucks, but you get the idea of what the character class can do.
Advanced Spy Tutorial [OMFGNinja]