!!! April 14, 2012: There is a newer post that summarizes my experience with this lens after two year of using it mainly for wildlife photography. Please feel free to click HERE if you like to jump over to the new post.
I found this lens under the Christmas tree last year. After three months of playing with the new "tool" I'm still at the beginning of a learning curve. However, the Sigma 50-500 has helped me so far to take some sharp pictures of birds and White-tailed Deer.
If you like to make images of wildlife, birds, etc., and your budget does not let you seriously think about the big Nikon primes, then this is the lens you should consider. It is not super-fast and the focus is not always easy to obtain in low light at the longer end of the lens range. But it produces sharp images if you provide good support and maintain a good long lens shooting technique. Zoom and focus ring have a nice feeling with a little resistance for good fine tuning.
What I really don't like is the lens creep. Unfortunately the lock works only at 50mm. Not a big deal, but I don't understand why little things like this have not been fixed after the first users complained about it many years ago. However, this lens has fulfilled my expectations so far, and I believe it has a very good price-performance relationship.
Nothing stands still and so has SIGMA just announced that the 50-500 f4.5-6.3 DG HSM will soon be available with Optical Stabilization (OS) function. According to their press release the OS function offers the use of shutter speeds approximately four stops slower than would otherwise be possible.
Spring has almost arrived and the first migrating birds show up already here in Iowa. I hope this lens will help me to make the images I have in mind. Let's get back to work…