The FD 55/1.2 is a beast, meaning it's big and heavy. Build quality though, is top-notch. Optically, because it uses ordinary spherical elements, it's soft wide-open. You'll need to stop down to about F2.8 to get a razor-sharp picture. Still, there's something undeniably cool in having this lens mounted on the front of a Canon FD camera.
It depends if you want to improve your photography or your street cred. then.