If you have never used a 500mm lens before, they take some getting used to. The size and weight is very different to say a 55-300, and you need to get your technique right (stance, bracing, grip, breathing etc). The longer the focal length, the more sensitivity there is to movement, so if you are shooting handheld at 500mm even with shake reduction on you need to use a fast shutter speed (at least 1/500th sec and preferably 1/1000th second). Of course (given the maximum aperture of f6.3 at 500mm) this means you need good light, or expect to use high ISO. Use a solid tripod if possible - this will let you use a slower shutter speed.
Like most zooms, this lens produces better resolution when stopped down one or two stops. Above about 320mm, the widest aperture is f6.3, so for best results you need to stop down to f8 or so. Another advantage of using f8 is that the greater depth of field is more forgiving if you have trouble focusing precisely (e.g. a bird in flight, when you just have to shoot and hope).
While on the subject of focus, I find my 170-500 auto-focuses fine. Of course with a maximum aperture of f6.3 it will struggle to focus in low light. Like any screw-drive lens AF is a bit noisy, but it isn't slow.
The 170-500 is much lighter weight (1.3kg) than the later Sigma models, 150-500 or 50-500 (each close to 2kg). Believe me, that 600g difference really matters when carrying it.
As for resolution and IQ (I have the non-DG version), the key is not to expect too much of the lens at 500mm. Don't expect National Geographic photos of small birds 100m away. But it does very well with subjects within say 20m or so. And it is very good indeed at the wider end - at 300mm it is a smidge faster than the 55-300 (f5.6 v f5.8) and the IQ is as good or better.
Some samples.