This lens is at the cheap end of telephotos, and its performance certainly depends on the users level of expertise. Starting at f/4 means lower shutter speeds, and handheld shots at 300mm on an APS-C camera can be tricky, but not impossible.
The build quality of this lens is fair, it's not a tank, but it's not plasticky either. It's definitely a happy-medium. The lens itself is light, which makes telephoto shots slightly harder but does make this lens couple better with lighter DSLR cameras.
A major niggle I had was that the macro switch would get stuck once engaged, and then the lens barrel would not go back to its original size. This means you need to switch back to manual focus, readjust the lens extension, and then you will be able to fit the lens back to its original compact size. This is a widespread issue as far as I'm aware, so I'll have to knock points off for that.
As far as image quality goes, this lens is neither good nor bad. At 70-200mm the quality is reasonable, but there is a drop off in quality further towards the 300mm end. Colours are vivid, thanks to the APO coating. And sharpness is as stated, good throughout although dropping towards the 300mm side.
The autofocus is fairly snappy but sometimes hunts for a little while, it's not silent either. It's not the loudest AF out there by far, and I wouldn't say this is a downside at all. Especially when you're working at around 100cm from the nearest subject.
Taking all into consideration, this is not a bad lens. Very far from it, in fact. I would recommend this to anyone starting out and perhaps in search of a first telephoto lens. But for enthusiasts and higher, I would suggest spending more and looking at better-quality alternatives. I would rate this lens 3.5/5 but granted it served its purpose and got me a few cracking shots I'll up it to 4.
I hope this helps!