I personally did not like the Sigma lens I bought, so that is a great suggestion, to rent the lenses! I must look into that myself actually. It was a 70mm-300mm lens. The Sigma lens was not as crisp as the actual (in my case Canon) Canon lenses I have. (18-55 & 70-200). The more I zoomed, the more of a blur would happen. (And no, not because I was moving, or the camera moved). I am sure that the more expensive lenses are made better though...and I know lots of people that do use them, but I have heard several of them also say they are not as crisp. (They use Nikons). Not sure if that was helpful, but maybe...
