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I won't say anything negative about the Sigma, since it's probably a pretty good lens, but contrary to what someone here wrote, the 17-55 is actually a stellar lens, one of Nikon's best. Take that to the bank. There are other good lenses, but the 17-55 is seriously good at what it does... and if you have the talent, can create great images. I seriously doubt that lens would be the limiting factor in most photographer's results. I won't tell you which lens you should buy, but don't be easily dissuaded by other's comments... just saying.