OK, a few more notes on this, now that it's properly blocked and all. Some of this hopefully answers your questions, desiknitter.
The pattern for the hexagon coat comes from Norah Gaughan's Knitting Nature. The pictures in the book suggest a coat that fits rather like my final version, I think. (Woah... while googling for a link to a picture, look what came up!)
When I first started to knit this, I realized that the actual dimensions of the coat were for something much bigger and, frankly, baggier. This is not what I wanted.
The pattern is, despite, that, very well written--the dimensions of my pieces were EXACTLY right, per the diagrams in the book. The construction of the hexes, which are really the most innovative part of the pattern, worked really well. So all I wanted to do for my second try at knitting this was to cut the whole thing down in size.
I did this by pinning my original pieces together (I'd finished the back and both sides, and had also knit a bunch of the hexagons onto the front pieces). Trying it on, and then pinning it down to a size I liked better. Then I mentally sketched out what that meant in terms of knitting.
I ended up with a couple of post-it notes stuck in my book, which included the deep and scary math (that's a joke) required to do this.
And then I jumped in. I had the first time through decided not to use ribbing on the edges, because I'm not that much a fan of ribbing, really, and it's not like the stitch pattern here would roll. I did, however, use smaller needles for the first inch or so to keep that bottom edge tight.
So, I cast on 118 stitches, placed markers for the sides, and went at it, knitting the whole body at once. This was, I think, a very good idea. In the original version, the back, in particular, got very, very boring to knit. This time, though, at least until I got up to the armholes there was always shaping at the edges, to make the indents into which the hexagons fit. That kept things interesting.
From this point on, this was really easy. The indents keep you on track. I sized down the armholes slightly. I knit on all the hexagons, and they, too, worked perfectly.
The sleeves weren't bad, either, though I dreaded them. I cast on on the smaller needles, did about ten rows of the pattern, then switched up a needle size for another ten rows, then started to increase at both sides every ten rows until the sleeve got broad enough for my upper arm. Then I continued until the sleeve was long enough. Then I shaped the end using more or less the same decreases called for in the original, but frequently comparing the sleeve cap to the armhole, making sure that I had the same length edge on both (this is tricky, because the geometry of this kind of sleeve involves non-matching curves).
Now, the yarn.
I used Laura's Granada, which is a lovely springy merino wool. Unlike a lot of Laura's yarns, it's essentially a single color (though it's got a strand of black running through the whole thing). But even it has some very nice subtle variations of color in it, which gives the whole thing some extra interest. It's really, really nice, I think.
I also quite like how it works in a sweater this length. In the first iteration, which was another couple of hexes longer, it got really quite stretched out when worn--the weight of the bottom pulled the top out of shape. I actually ended up doing one more hex repeat on this version than I expected to do--based on the stretched out version, I thought I'd only want three indents before the armholes; in reality, with the springier version, I did four.
That's the only thing I'd keep in mind as far as fiber choice. I thought of doing this in a wool/silk blend, and I actually do think it'd work reasonably well. It is quite a nice shape done this way, and I think it could easily be dressed up or down based on the yarn you use. Mine is, I think, nice and basic (it's also a little more brown than these pictures suggest--the sun bleached the color a bit when I was taking pictures outside yesterday, even though I used an umbrella for shade), and should let the coat/sweater be dressed up or down; dressier yarn would really make it pop, though.
Because the shape of the coat itself is so basic, I think you could easily substitute yarns of all sorts, as long as you're willing to do a bit of math to figure out where you want to start. The only issues to be faced in that case have to do with the hexagons. I assume that if your gauge dimensions are proportional to those called for the hexagons will knit up well. If they're off significantly, though, you might have to tweak the hexagons to get them to knit flat.
There's also the issue of the armhole. You have to end the fronts at a particular place in the hexagon indent pattern, so you have to start the armhole shaping so that you'll have the right amount of length when you get to that spot--if it's not enough, you'd have to do an entire additional indentation, which might be way too much.
So, once again, I'm very happy with this sweater, and I think that it'll get a lot of wear. In the fall. Because now spring is springing, and... well, I might get a couple of wears out of it before summer... summs?... but, basically, this will be a staple come fall.