Conventional wisdom says that primes (lenses of a fixed focal length without the ability to zoom) have better optical quality than zooms, and that's almost always true. But in a few rare cases, zooms can be better than the available primes (see Nikon's incredible 14-24/2.8 for an example), and in others the difference in quality is so small that it becomes functionally irrelevant. So it is with this lens.
Both Canon and Nikon have phenomenal "long guns" in their lineup: 200/2, 300/2.8, 400/2.8, 500/4, 600/4. Each of these is bigger, heavier, and more expensive than the last, with the 200/2 and 300/2.8 being roughly 6 pounds and $5,000 while the 600/4 is closer to 11 pounds and $10,000. And they all deliver amazing image quality with near-instant autofocus performance... but none of them can zoom. And to cover the 200mm to 400mm range, you'd need to spend $18,000 and carry around nearly 22 pounds. You'd get lenses with larger apertures and could either shoot faster or in less light... but if you can live with f/4 (and I can absolutely live with it, even for wildlife in Botswana or airshows at dusk), then you can get that range for $6,000 and 7 pounds, or add in a 1.4x TC-14EII teleconverter to get a 280-560 f/5.6 lens that is still stunningly sharp.
If you're making enough money to buy the primes, and you don't miss the ability to zoom, and you're willing to carry two or three big primes around with you (along with 2-3 bodies to put on them), then this lens may not be attractive to you. But for everyone else, who either cannot or will not pay $18,000 and carry 22 pounds of glass, this lens is a bloody godsend. The image quality is incredible, the AF performance is extremely fast, and there is not much of a difference in either IQ or AF from this lens to any of the big primes. A little, yes; but not a lot. And no other zoom on the market, from any company and at any price, can offer the speed and quality this one does.
I've used this lens since 2007, on three different bodies: the D200, D300, and now the top-of-the-line 24-megapixel beast of the D3x. Even on the D3x, this lens provides more resolution than what the camera can deliver. Don't try this lens if you can't buy it... it's absolutely addictive. The one thing to keep in mind if you're moving into the "big guns" is that 7 pounds does not sound like much, until you're holding it up with your left hand for a few hours, a foot away from your face, and trying to hold it steady. Even with VR, long lenses do take some practice, some strength, and some technique; so get lots of practice, and don't expect to be able to get the very best possible quality from this (or any comparable) lens in your first few days or weeks with it.