I'm looking into buying a longer lens for wildlife & mild sports. I know by looking on here some folks have gone with some Sigma lenses because they are less expensive than their Canon, Nikon, Sony etc line and I'm asking how have they been as far as durability and longevity are concerned? My current lineup is a D7000, lenses are 70-300mmVRII and I started out with the 18-200mm Nikkor. I'm looking at lenses in the range of 400-500mm. No primes as I'm trying to keep cost down since this isn't my income means. I have been reading up on the Sigmas be it the 120-300 f2.8 with a teleconverter which is high on the price side at this time, or their 50-500mm as it seems to have better reviews and newer guts than the 150-500mm from them. I looked into getting a Nikkor 80-400 but it seems like it's really needing a refresh at this time or there are better options out there. I also looked into the 80-200 and adding a 2.0 TC but it seems like that option is ok for a quality setup. I do feel a bit limited by the lineup Nikon has at this point so again how are the third party lenses holding up at this time for those using them? Thanks!