After my failed experiment with the 600mm f/4, I was a little tentative about the 400. While quite a bit lighter, this is also a large, intimidating lens. This time around, I made sure I had the prerequisites - the Gitzo Systematic Series 3 tripod legs and a solid gimbal head (mine's from Calumet Photo).
I can't overemphasize the importance of a good tripod/head combination to support this lens. In addition, lugging this 25-pound deadweight around requires a commitment to frequent shoulder and neck workouts! A lot of people have asked me whether the final result is really worth lugging this setup around, and after 2 years I can say that the answer is a YES! The lens seems to grow smaller every time you use it and it really isn't a big deal any more.

Build Quality & Handling
Nothing groundbreaking to report on this front - as befits a lens that costs as much as a small island, the build quality is fantastic. I did have an issue where the autofocus completely stopped working, but to Nikon's credit they fixed it for free even though my lens wasn't under warranty. Since then the lens has been a model of reliability. I almost always use this on a gimbal head or a rest, so handling isn't an issue in my opinion. I've tried handholding it, and take it from me - it's impossible. No matter how often you hit the gym.

Image Quality & Performance This lens delivers amazingly sharp pictures with good support and long lens technique. Bokeh is unbelievable and really makes the subjects pop. I coupled the lens with the Nikon TC-20EII 2X teleconverter for the first few months that I had it. The results were good but not great - I felt that the compromise in image quality was a little too much. Upgrading to the TC-20EIII teleconverter made a world of difference; I stop down by 1/3 to 2/3 stop for an effective aperture of f/7.1 and the resulting images are tack sharp. Check out the detail on the Dragonfly's wings below (this is a crop of about 10% of the original image):

Eastern Phoebe with Dragonfly

This is also probably the fastest focusing lens I have ever used - it's fast, sure and is a dream to use. However, I rarely use this lens without the 2x teleconverter, and adding the teleconverter sometimes makes the AF hunt a little in low light. However, this and the Nikon 300mm f/2.8 are probably the only lenses that can handle a 2X TC with such ease.
Here are my contributions to the big VR argument - I haven't attempted a scientific test but from what I've seen, using the VR in NORMAL mode it doesn't seem to hurt the images any! How's that for a cop-out?! Seriously, the only time I switch off VR is when I'm doing video - VR makes it sound like you're shooting from inside your local laundromat.

Conclusion
This is one serious lens - both price wise and in the commitment required to use it. However, all this pales into insignificance when  you finally snap that picture of a beautiful songbird in perfect focus with the background a soft, dreamy wash of color. For anyone who hasn't used one of Nikon's big lenses, here's my recommendation : rent one - you'll fall in love and eventually buy one!