Ditto on the life-changing quality of AGS - without even having released a game yet, simply using the engine started my career in games journalist, lead to a job at EA and got me an awesome girlfriend. Though I probably shouldn't vouch for AGS' miraculous qualities without adding the disclaimer: "Results may vary"
If you're worried about cross-platform compatibility, check out the very promising XNA based AGS port
XAGE, which could very well offer Mac, Linux, browser (via SilverLight), Xbox 360, Android and even iPhone/iPad support by the time your game is done.
