Anyone use one of these as their main lens? I'm currently using a D3100 with the standard 18-55mm lens and 55-200mm zoom lens, but find myself constantly switching between the two, I'd also like a bit more length as well (on the zoom ) So I've been looking at the various offerings from Nikon and the Sigma ones that are compatible. At £369, this seems an absolute steal compared with the £849 price tag for Nikon's own brand. So the obvious question here for anyone who has used either or both of them, what is the compromise from buying the Sigma as opposed to spending nearly 500 quid more on the Nikon? I mostly use it for day to day stuff when out with the family, sporting events as well, but also some night photography. Any advice oh wise SMB?