I have a longish relationship with the 300mm ED IF ais on a D800E, and can vouch for it as a affordable useful lens that offers high IQ.
It really is a joy to use in a controlled environment like a hide. To get the most from it, I recommend having it CPU chipped.
My experience to date is with the unreliable Dandelion CPU, I have installed two onto the same lens, they are fragile
and my ones did self destruct over a period of time caused only by exchanging lenses.
There are better options for a CPU available through NG.
For me the opportunity to focus trap with the lens as a shutter release signal was great, offering many successful keeper images.
The option of a sub command dial aperture control is a good support for keeping a consistent exposure,when working with fast shutter speeds.
On a Gimbal, with a remote shutter release attached, it is possible to nail BIF images at about 25 metres using focus trap,
if you become familiar with a re-used flight path at closer focal length, you can get images at closest focal length, I,ve captured Tits about
4 metres from a hide, and a kingfisher about 6 metres out on a perch. These are bird species that do not pose for the camera when disturbed. At these distances you a disturbance. I have a few all time favourites of Perched Kingfishers whilst stalking along a river at about 35 metres.
To give a idea of a budget and if this option can be seen as "Worth a Punt". I will include a few figures Nikon 300mm ED IF ais ebay £ 360,
Dandelion Chip $ 30. 32 teeth per inch metal blade pack of six £ 4.00. Fine metal file £ 3.00. 2 Part Epoxy Glue £ 4.00.
A fews hours reading will supply much knowledge. A more robust and dependable CPU Chip from NG is approximately £ 90 with jig hire included. I,m not 100% sure of the skill set required for this as it is my project in waiting.
It is affordable to achieve, but a period of hunting for the correct price lens will not make it immediately available at the prices quoted.
I have never compared this lens to another modern Nikon equivalent, but have seen its end results compared to images taken on a side by side photography trip to a Hide at a Nature Reserve, where the other camera had a very modern Canon 300mm f2.8 attached.
The outcome of that trip planted the seed, that led to the Canon owner now being a owner of a D810, 600mm f4, 300mm f4 PF.
I think it is fair to say the 300mm ED IF ais + D800E left a impression when images were shared at a later date.
As always Beauty is in the eye of the beholder, so I can't vouch for this being the response from any other camera brand owners.