Nogbad
This site is very useful for gauging the quality of lens
http://www.photozone.de/active/survey/querylens.jsp
It is one list that contains most lens.
It only takes a few seconds to gauge if a lens is poor, average or good quality.
Then you know what you are getting instead of being surprised pleasantly or not.
It's not Rocket Science
You will see the Nikkor 60mm scores much higher than the Sigma 50mm