I LIKE MINE...
I had a "nifty-fifty" for about eight hours, 'til I bonged it gently on a door frame while carrying it on a camera over my shoulder. front element ended up spinning on the floor. So much for a nifty fifty.
I like the better build of the Mark-I (but, I haven't tested it against a door frame). It's a fairly good lens wide open and gets a lot better by f/4.
However, since I got my 17-55mm f/2.8 IS lens which is a pretty viable low light lens; I haven't been using my Mark-I and will be selling the Mark-I and a older 28mm Sigma f/1.8 lens. Like a lot of fast, inexpensive prime lenses, these two lenses are just O.K. wide open and don't really start showing better IQ until they are stopped down to f/2.8 or even f/4. My 17-55mm IS lens is sharp wide open and the IS makes up for the one stop slower aperture. Of course the 17-55mm f/2.8 lens is expensive but, it can replace a myriad of primes from the 20mm f/2.8 through the 35mm f/2 and the 50mm f/1.8 (either flavor).
