The 50L had (and maybe is still a problem for people, although not for me) the back focus issue, but the Sigma also had some focus inconsistency (again, maybe this is better on later models and non-existent for most users). Both lenses are fine for most purposes. I know at least from my personal experience that Sigma lenses in the past had some focus inconsistency. My favorite lens on my 7D was the Sigma 30mm f/1.4. It had beautiful image quality and a great build, but the focus was very off and slow much of the time, which is why I haven't bought any Sigmas since. I think that a lot of recent effort by Sigma has been put into making their lenses top-notch though.