It's fast, it's long, it makes great images, and... it's heavy.
The 400/2.8 G VR was first introduced in 2007 and in production to 2014. With over 7000 units made, it's safe to day that this is the most popular (by numbers) of the 400/2.8 primes that Nikon is made. Still a relatively new model (succeeded by the 400/2.8E FL), there are many copies still "in the wild". Price wise, they can be the sweet spot on the used market, landing under the current version at $11000.
A 400mm/f2.8 lens a must have piece of equipment for any professional sports photographer. The 400mm focal length is suitable for a number of applications, and the 2.8 aperture is needed for the extra light needed for night sports and for the definition of subject from a background. It takes a Nikon 1.4x TC III very well, and makes a close equivalent (560mm/f4) of the 600/4. I barely notice any change in the AF speed in good light and don't see really hardly any loss of image quality. It's also takes the 2.0x TC III fairly well to become an equivalent (800/5.6) to Nikon's 800mm/f5.6. Auto focus does slow down somewhat and I do see some loss of image sharpness. However, doubling the focal length of the lens and still having a f5.6 aperture makes this a very versatile lens. Use this combo with a DX body like the D500 and you've got some incredible reach.
So what's not to like about it? It's heavy. It's nearly 10 lbs. One advantage of the replacement FL version is it shed nearly two pounds. It's also a physically big piece of gear and takes some technique adjustments and if you've never used a super tele before, it'll take some getting used to. The price is also a barrier to many, but it's a bread a butter tool for any professional sports photographer.
On the last point, an earlier version (AF-S II) can be had for significantly less on the used market. Keep in mind though that Nikon guarantees to keep replacement parts for a lens for seven years after it' falls out of production. The 400/2.8G VR should be good then at least until 2021, and with the number of this lens produced, even further beyond between Nikon and 3rd party repair facilities.
Since it's been out of production since 2014, to acquire one will have to be through the second hand market. Beyond the general condition and functionality, there are two things to look for that might not be apparent if it's the first time you're buying one. The first detail is that the lens uses rear filters. The lens comes with a 58mm clear filter that's a particular model from Nikon. This filter is part of the optical formula, so if it's not included, look into the availability of finding a direct replacement. Secondly, check out the condition of the lens hood. It's a two piece item and can be a little susceptible to wear and tear. This is important because you'll find yourself standing the lens on the lens hood quite often and you want it to be stable. If you have to replace it, them last time I checked, it was over $500. That's something to take note of.
10 years old and still a very relevant and useful design and piece of equipment. You'll see many of this model at pro sporting events and they produce images you see in the media daily. If you need it, there isn't anything that's going to fill the shoes of a 400/2.8G VR (except the newer FL :) ).
Update: I sent the lens into Nikon because I noticed a focus anomaly. NPS (Nikon Professional Service) sent me a 400/2.8E FL to try and use while my lens was being worked on. I loved the E version. The lighter weight was really nice but the balance of the weight took some getting used to (one three hour MLB game). When I received my 400G back, it focused faster and more reliably and was sharper. It came back amazing. The sharpness and responsiveness is off the charts. Besides the aforementioned weight and balance difference, the only other difference between the two lenses was with using teleconverters. With any lens purchased second hand, I recommend sending it in to Nikon for a check up.
I now use my 400G with a 1.4x TC III at MLB games - pretty critical stuff. Sharpness and focus speed are great. The 400E had just a minuscule advantage with the TC. I don't think most people would notice unless they did the extensive testing that I did , including dialing each lens for focus tuning.
At the end of the day, it comes down to the price difference. Both lenses bare are equal in my tests. The difference is the weight (big deal for football), balance and a small difference when using TC's. My plans were to upgrade to the 400/2.8E FL as soon as possible as it's my most used lenses and I like the latest and greatest. However, it's now going to take a little more time to move away from my Nikon 400/2.8 G VRII.