Nice Tim...looks like you'll be joining the Spy Brigade - now you can shoot stuff so far away they don't even know they're being shot. Works great with wildlife, street candids, and those distant things that you'd otherwise miss. Lots of fun!
Dan...not that my test is anything official - but in testing and comparing the 150-500 Sigma and the 200-500 Tamron, I felt like the Tamron had an edge in sharpness, especially wide-open, and in color. The 50-500 Sigma seems a touch better than their 150-500...I considered that one too, but the Tamron was lighter and more portable (Comaratively, of course...it's still a tank!). I've a feeling the shorter the telephoto range, the better they seem to be able to get the optics right throughout the range - the 50-500 is a bit of an exception, but is so huge and heavy...otherwise, the 120-400 Sigma doesn't have to cover as much range, and therefore seems to be a bit better tuned throughout the range. I found the same to be true of the Tamron 200-500. And since when you use such a large, heavy telephoto lens, you're unlikely to keep it on while shooting 50mm or 100mm...why worry about covering that wide range?
Tim...looking forward to seeing some samples from that lens!
