I've been meaning to do this review for a while now, but just never got around to it. I used the Sigma 50-500mm f/4.5-6.3 (also affectionately referred to as the 'Bigma') for around 3 years. Mine was the non-stabilized version; from what I understand the newer stabilized version (OS) has exactly the same optics.Build Quality & Handling
For such a (relatively!) inexpensive lens, it's built like a tank! There's very little plastic on this lens (limited to switches and lens hood) and the overall feel is that of a much more expensive item. The zoom ring was a little tight when it was new, but loosened up after a few months of use. The downside of this was that the zoom would 'creep' when pointed downwards. I also can't say I'm a fan of Sigma's 'furry' finish, as it it marks and scratches pretty easily.
The biggest problem I had with this lens was not really a fault of the lens - it was a design compromise. This lens is big enough and long enough that you want to put it on a tripod with a gimbal head when using it for any length of time. However, since the lens changes length when zooming it changes the center of gravity, making it hard to balance on a gimbal head. Image Quality & Performance
The image quality of this lens is pretty decent for how much it costs. In the end, you buy a 50-500mm lens primarily for the reach it gives you, so most of my comments address the 500mm end. The maximum aperture of f/6.3 is a little limiting with low light levels, but it's not too bad. Stopping down to f/7.1 or f/8 gives pretty crisp images. Autofocus is pretty snappy with the HSM motor.Conclusion
All in all, this is a great lens for the price. I bought mine for around $1,000 before Sigma introduced Optical Stabilization (OS) and it served me well until I moved on to bigger and better glass. The addition of OS has increased the price of this lens by around 50%, so given the choice again I'd probably still go for the non-OS version as I mostly shoot off a tripod or rest and don't need the OS.