I bought this lens knowing literally nothing about SRL photography. I had just bought the Rebel XT/350D kit and thought I needed a new lense since everyone else commented that the EF18-55II was doing a diservice to the camera. This lens was affordable, had a 4.4 MTF grade and, hey! it was the reference lens used to test even the very best cameras in some famous websites! So I could not go wrong with this, right? well, right and wrong...
First thing you should know about this lens is that it's an interiors / available light / night shooting piece of kit. In high light conditions, this lens is more trouble than it is worth. Blown-out highlights are the name of the game. Even under low light conditions you have to be careful with the apperture. I remember some shots I took in the Christ Church Cathedral (Oxford). There were some beautiful white roses in one of the chapels. When I reviewed the shots at home I was horrified. God, those white roses literraly glowed in the dark! The lens is THAT light-sensitive wide open.
And then the DOF... If you are going to buy a super-fast lens first make sure you know how to focus, or at least know what you want in focus. Wide open, a couple of cms are all that it takes to blur things out. I have some portraits in which you can really tell that the nose is focused, while the cheeks are not!
The 50mm focal length becomes 80mm on a 1.6x sensor, making it almost useless for architectural photography, unless you intend to travel to Paris or some other "open-plan" city. Forget places like Lisbon.
So, in sum, I was very disappointed, until I started to understand what I was doing wrong, and started to correct all the problems I had encountered.
This lens has taught me some photography and for that alone it is my current favourite. An excellent portrait lens if you can control the DOF, terrific bokeh if you are a fan of that. And finally, the price. No matter how many times you get it wrong. This lens will eventually give you 1, 2, 10 or 100 wonderful pictures that will alone justify the price you paid for it.