This fast lens is expensive because it is extreme. It is very difficult to make lenses this fast - if it were easy we'd see lots of f1.0 and f0.8 lenses. The former is extremely rare (Canon only I think) and the latter doesn't exist to my knowledge. Regarding the Pentax lens in question, you are not paying for additional quality, you are paying for additional, cutting edge speed. If you don't need it, don't buy it, you're wasting money. If you really need the speed then you have no choice. The quality wide open isn't fantastic but you're likely to be using fast film and on the edge anyway. As you stop down it compares okay to the other Pentax 50mm lenses. Difficult to rate as it's one of a kind.
A lot of people recommend getting the 1.4 and 1.7, but it really depends on what you're going to be using it for. I've owned an embarassing number of Pentax 50mm lenses, and I've never been able to seperate the prints from different lenses. When I know I might be making larger prints, I use my "best" 50mm lens, but that's for my confidence not because I've ever been able to tell.