I have heard extremely good things about the Tokina brand. I have not used them myself, but would not hesitate to use their higher end lenses. I would use their lenses over Tamron or Sigma any day... I have used the cheaper 70-300 Nikon before and returned the lens 2 days later if that tells you anything, I don't know how this would compare to the Tokina or not... you really do get what you pay for when it comes to lenses, get the best that you can afford.
If you are looking for excellent quality and want the long telephoto range I would highly recommend looking for a used Nikon ED-IF 300mm f/4 (not the AF-S version, unless you can afford one) I owned this lens for several years and the image quality is superb... one of the best bangs for your buck in my opinion, you should be able to find one in the $400-$500 range... seriously, I HIGHLY recommend this lens to anyone that is looking for a good telephoto at a resonable price.
If you would like to see some sample pics go to
www.photosig.com and register. Then you can search photographs by lens, they have an amazing database of images.
Let me know if I can be of any further assistance :D
Kevin.