As a Sigma lens user I can assure you that they perform great and cost a heck of a lot less than the camera makers lenses. I have 3 of them so speak from experience. The 18-50mm lens will not do well for macro shots. I would recommend the Sigma 150mm Macro lens because I bought the 70mm Macro lens,(app £500) and I wish I had bought the 150mm as it allows you to be not as close to your subject, thereby not scaring it off. Go for IS with Sigma, (Image Stabilisation) which will give you a sharper pic. For a zoom I would opt for the Sigma 18-250mm lens with IS, it scored a 91% in WDC Mag test. All the best, Mike.
Humility is an endearing quality and gains many friends, whereas arrogance loses them. Mike, 2012