500/4 IS is a wonderful lens... A good, solid tripod and gimbal head are pretty much a mandatory accessory. I recommend anyone considering the lens also budget another $1000 to $1500 for a tripod and head, if they don't already have one.
I suggest trying the Wimberley Sidekick with a hefty ballhead (I've been using Sidekick with Kirk BH-1 for about ten years). That way you don't have to change out the tripod head to use it with shorter lenses and camera plates. The Sidekick works well with the 500/4 and 300/2.8. I suspect it would be adequate for the 800/5.6 too. The 400/2.8 and 600/4 are the largest and heaviest of the super teles and might need the full gimbal head (and a somewhat dedicated tripod to use with it).
The Sidekick is handy in another way. You can mount a camera with a camera plate vertically on it, so you don't need an L-plate.
I wouldn't be surprised to see the IS upgraded in 300/2.8, 400/2.8, 500/4, 600/4 and 800/5.6 over the next year or two or three... But I doubt there will be much improvement other than that. Frankly, there doesn't seem to be a lot of room for improvement.
400/5.6 might finally get IS (which it needs).
300/4's IS might get upgraded, too. It's the oldest IS lens in the line-up and could use a refresh.
I've never used it, but have seen some really great work done with the 400/4 DO.
Problem with the 500/4 IS, now I want the 800/5.6 too!
Have fun with the 500/4 IS, it really opens up new opportunities.
I'd be better off with a drug addiction. 
