It's not just me saying this, time and time again the 50mm f/1.4 lenses from Canon, Nikon, Minolta (Sony), Olympus and Pentax are put against each other (although, now Oly no longer makes it's offering due to it's four thirds system). The result, the FA 50mm f/1.4 pretty much always comes out top or close to top. It's also cheaper than any of the others, often by a country mile. It has an aperture ring, meaning that it can be used on any Pentax camera since the evolution of the K mounnt (circa 1975) with no compatability issues. It's brilliant, right! So, why not five stars? read on...
As with pretty much all fast fifties, the lens is 'dreamy' wide open rather than tack sharp, sharpness falls off in the corners quite a lot (not quite such a big issue on digital) and it only starts to get very good corner to corner at around f/5.6 (on my copy). It's construction is plasticy, the manual focus ring is better than a lot of AF lenses but still nothing on a good 'ole metal MF lens.
That said, if you are operating in marginal light an extra stop or two worth of light can really mean the difference between a shot and no shot. It's a lens that deserves a place in anyones bag, and can get you out of holes other lenses couldn't even see. So long as you recognise it isn't a miracle cure, you should be able to use it, enjoy it and best of all, take some great photos with it.