If you’re looking for a high quality telephoto zoom, look no further than the Tamron 70-300mm.
Generally speaking, it’s about the same as the Sony 70-300mm F/4.5-5.6 SSM G, but costs half as much, and currently with rebate it’s $399. While I find the build quality and optics on par with the Sony 70-300mm G, the Tamron’s focusing system (on my copy) is not quite as fast or accurate, and the Tamron omits the often important focus hold button and focus limiter switch, that’s probably why the Tamron costs half as much. Check out the review for more info.
Look for a quick comparison between the Tamron and Sony 70-300mm lenses in a day or two.