I have it on firefox 0.9.2 and it doesn't fill me with confidence as it is easy to spoof the SpoofStick.
Yes it trys to identify the origin of the true website, but I found a simple exploit of SpoofStick totally invalidating this check.
I regularly visit a website, using a URL domain redirect. I type the friendly short domainname.com and it redirects you to thenotsofriendlyllongdomainname.com website, directory and page.html, etc.
How? the redirect can use frames as the redirect so the website opens inside a frame that appears to be the domain that you typed but it has been redirected. This in hosting terms is known as URL masking and negates SpoofStick.
When you enter a domain check the status bar at the bottom left of the browser window and you can see the path to the true website, where it is downloading images and web pages, etc.
SpoofStick is useful, just don't think it is 100% and carry out the above check when visiting websites from webpage or email links.