Looking at a Sigma 120-300 f/2.8 EX APO IF HSM Autofocus AF-D. Is this a good lens? Can I use my Nikon 1.4 teleconverter with it? I'm trying to find an affordable zoom. I bought the Nikon 300 2.8 VR but had to return it because of a problem with the focus. Now I'm looking again, but for something maybe not quite so pricey. Is the Sigma worth getting? Thanks!