Well, if nothing else, this thread has encouraged me to get to know the FA77 a little better. I took some time this morning and shot several more images with it. Not necessarily exciting stuff, but I put more effort into picking out stuff that might yield interesting pics. Sorry, no more comparisons, I only had one camera with me today. I added about 20 more shots to the Flickr gallery I linked earlier in this thread, and I'll post a few favorites below.
The result is that I believe more than ever that the FA77 has a uniqueness about it which is somewhat identifiable, very repeatable, and I feel like if I used a different lens, the images would not be as nice. Call it what you will.
It also occurs to me to argue just a little with the point about a symbiosis of photographer and lens that arises from a shooter's familiarity and experience with using the lens. In my case, that is certainly not true: I am pretty new to Pentax, and while I like the K-5's small size and I am generally happy with the image quality, I can't say I know the system well, and certainly can't say I'm in love with it. Compared to my D3, the ergonomics are damn clunky. The feel in my hands is not as solid. The FA77 is clearly a well-constructed lens, but the pleasure of focusing it pales in comparison to my Zeiss Makro-Planar, and yes, even in comparison to my $100 Nikkor 105/2.5. If I am looking for a comfort zone that guarantees me I will get the quality and the files I want, and not miss any shots fumbling with settings, I grab the Nikon, every time. I'm not saying anything negative about Pentax, and it's largely an effect of me just being very familiar and comfortable with the Nikon setup; I know I'll get more comfortable with the K-5 over time, and the FA77 will encourage me to shoot Pentax more often. But not because the lens "feels" right to me or anything. Because I've got things that feel a lot more right. So in my case I can rule out my "love for using the lens" as a factor in getting better images with it than I might get with some others.