I was shooting with a pair of 40D Canon DSLR's before the D300 came out. Prior to that, it was a 5D and 30D combo. Prior to that it was a pair of 20D Canons, and before that it was a Nikon D200 and D70 combo.
It was the D300 that brought me back home to Nikon.
The D300 was a revolution in many ways. The D300 offered everything that the previous Nikon flagship (the popular D2Xs) had, and more. Even more amazing, the D300 had Nikon's new industry leading Auto-Focus system, the same one found in the D3! How could this be? No camera maker had ever before put their top of the line AF system into a DSLR at this price point!
The D300 had a rugged, professional build, metal body. The D300 came loaded with a brand new 12 Megapixel sensor that was much cleaner at high ISO than any previous Nikon DSLR (except, obviously, the D3 which was itself a new camera introduced at about the same time as the D300). The D300 offered more features than ever before, and it offered all of this at the un-heard of price of $2,000 Canadian! The camera world was shaken! A camera that did everything better than the D2Xs, at a third of the price!
My D300's have frozen my camera equipment expenditures. Amazingly, I haven't bought a new DSLR in almost 2 years. Even more amazing, I have no plans to purchase one any time soon. The D300 did for me what the F100 did for so many people back when we shot film. It met all my requirements, and then some, leaving me no burning desire to buy any of the latest and greatest that have come since. Granted, the D300 is still fairly new and has most, if not all, of the current Nikon technology, so it's not like I'm missing anything.
What is new for me is the contentment. Previously, I've always been eager to try out the newest DSLR's that are released to see what they can do (and I have). Now, not so much. My D300's perform wonderfully and meet all my photographic needs. The 12 Megapixel sensor inside the D300 is fantastic, and certainly more than enough MP's for all but the most demanding of jobs. Even the incredible D3 and D700 are 12 megapixel, although granted an entirely different 12 megapixels.
The big difference between the Nikon D3 and Nikon D700 sensor versus the D300 sensor is the high ISO performance. The D300 has very good high ISO performance, but the D3 and D700 take high ISO performance to a whole new level. NOTHING can touch them for high ISO performance. Shooting at ISO 6400 is perfectly acceptable with the D3 and D700, whereas, I like to stay at 1600 ISO and below with the D300. Sometimes I'll kick it up to 3200 in a pinch, especially if I know the image is going to be black & white, but basically I stay at 1600 and below when I'm shooting my D300's.
Obviously the other big difference between the D3 / D700 and the D300 is the sensor size. The D3 and D700 are full frame FX chips, while the D300 is a crop sensor DX chip. Full frame isn't such a big deal to me, but for some it is nothing short of the Holy Grail. The big appeal of the D3 and D700 for me is the high ISO performance. They can practically see in the dark. I must admit, I do like the idea of having that SUPER clean high ISO option available... I just don't like the price. :)
While the high ISO performance of the D3 and D700 is appealing, it's not enough for me to justify the added price. The D300 has the same body and auto focus system as the D700, and the same number of Megapixels. The D300 has excellent high ISO performance, just not the Super Human high ISO performance of the D700. As such, for me at this time, there's just not enough there to justify the added expense of the D700 over the D300. The D3 isn't even a consideration for me as I prefer the lighter, smaller D300 / D700 body style to the larger, heavier D3 / D3X body style.
In a nutshell, the only real difference between the D700 and D300 that is a consideration for me is the high ISO performance. The full frame versus cropped sensor isn't really a factor for me as I like both, although I know for many shooters full frame alone is worth the switch to the D700 (or D3 or D3X). I'm used to DX, and I like shooting with DX cameras. That said, I also like FX, and I know I will acclimatize to full frame very quickly when the time comes.
So I'm left in a very comfortable, very foreign position. I'm VERY happy with my D300 Nikon bodies and have no plans to replace them in the near future. I've decided to pass on the D700 for now. In a year or two when the next round of Nikon DSLR's come out, they'll likely offer a whole host of exciting new features, and I'll probably buy one of the new offerings. In all likelihood, the D3X chip will be available in a D700 type body by then (probably a D800, but who knows for sure), and the price will likely be equal to, or lower, than the current D700 pricing. THAT will be a camera I will be SERIOUSLY interested in. Perhaps it will even have the high ISO performance of the D3 and D700, but with the stunning 24 Megapixels of the D3X!