Now that iTunes Match is available (the $24.99/yr) digital locker offered to iTunes users, you may be wondering how it all works -- before you spend the cash. Ars has a great article, but here is my experience.
My 8,000 songs just finished matching and uploading, and so far so good. iTunes matched over 6,000 of these, and then uploaded the remainders that it couldn't match to it's iTunes catalog. I let it run overnight and by this evening it was complete. Now I have access via iCloud to all of the music that was on my iMac (my full library) on my Macbook Air, iPhone, Apple TV, and iPad. Performance is fine on my Macbook Air with the music starting in under 3 seconds. On my iPhone, it typically takes between 5-10 seconds - which I think is much too long. If you start on a playlist, it's almost seamless, but the initial start of the first song is the one that takes the longest - frustratingly long. Petty, but when you get used to instantaneous playing (Spotify, RDIO, etc) you get spoiled.
Ars Technica has a great article worth checking out: