I wouldn't call the Nikkor 70-300mm f4/5.6 "pretty close to junk". Of course, compared to a f2.8 lens the quality is not as good, but remember the price difference. Lenses that cover such a wide focal length usually tend to loose sharpness between 200 and 300mm.
If I can afford it I would always try to get lenses from the same brand my camera is. They are specially designed for your camera, while Sigma, Tamrom and Quantaray are made for several brands. These lenses used to be cheaper than the original Nikon, Canon, Minolta etc. ones, but now the price difference seem to become less and less, so they are no option for me any more.
Frank Unterspann - Hamburg, Germany