I think that the dumbhead style shave horse that allows for the head assembly to be adjusted both up and down and forward and back, with an adjustable foot treadle is the most versatile and comfortable style around. Here is John Waters (an Aussie pal) sitting at my favorite style. You can find plans for it here.
There is another style of shavehorse that I run into a lot, and frankly, I'm not a huge fan. It's not that it doesn't work, it's just that I see more folks working awkwardly or getting frustrated while on it. I don't bring this up to be disparaging, but because I found that some subtle changes might improve it and if you have one, you might consider them.
While in Melbourne, where all of the shavehorses were like the one below, I noticed that the angle of the platform that the workpiece rests on seemed a bit steep. If you look at the wrists of Wayne in the photo, you see that they are out of alignment with his forearm, and his forearm is out of line with the spindle. This means that he must constantly adjust his wrists to keep the drawknife shaving consistently. Not a recipe for success, or comfort.
So I bullied my friend Bern into chopping up his newly minted shavehorse to try a hunch. I like a 10 degree slant to my platform. When Bern did this, the change was notable enough to justify chopping into the other 11. Look at Berns body position in the image below.
Not only is he a fine example of an Aussie craftsman, but he looks good, and comfortable doing it! You might be saying, yeah, but what if you are super tall? Well, lucky for us, they grow em huge over there and we had a 6'7" student who agreed that the shallower angle was a welcome change. Go figure.
Another change that you might consider if you are riding one of these, is to make an adjustable foot treadle. It wouldn't be too tough and I think that you'll notice that your energy is better spent when the treadle is properly adjusted. I suppose that the concept of the horse and the platform that ratchets up is supposed to make this unnecessary, but I'm just going from experience of watching folks use it.
And finally, I've notice that on lots of these, the platform extends too far forward and makes working on small pieces or near the head tough. So chop it off.
Here are a couple of ideas that I picked up from my friends Down Under that I will be putting to good use.
I've been meaning to do this to my continuous arm forms for some time, but seeing as they beat me to it, I give them full credit. If you are unfamiliar, the form usually has a tighter bend and a piece of angled steel that holds it in position away from the form. But since this is the position that we want, why not just make the form to fit!
And here are their wedges, with the clever cut out to make knocking them out easy (no more tapping the thin edge!).
Here is a seat carved from Kauri. It's pretty hard compared to our pine, or even poplar!
Not all of it was this figured, but it sure made for exciting carving. They assured me that it was a soft wood, but then again, they kept warning me about drop bears and hoop snakes as well!