If you want to stream any audio that comes out of your Mac around your house (or office, or party) this the app you need. iTunes can send music, but is limited to that. Airfoil can highjack any application, like LastFM, a mediaplayer or even your browser. Yes there is a slight delay the first time you connect to your Airport Express or other device, that is because it needs to establish a connection before it can send data, just like iTunes.
Having problems? Rogue Amoeba's support is excelent. Their respons very fast.