Been lurking around here and there, and the overwhelming consensus is that kit lenses are junk. What makes them junk? I mean, I've seen the kit lenses selling for around $250, while you can pick a Sigma or Tamron zoom lens up for half that. Does that mean the Sigs and Tamrons in that price range are junk as well? I ask because I don't really understand what makes the kit lenses junk, as I can get some decent shots with it, however, I need to get a zoom lens, and I had my eye on one of the Sigmas or a Tamron, but they cost about $140... Should I be saving my money to get something else?