OK, I'll admit it. I carry one. I have a stool setup too, but mainly only use it for the larger shoots. I'll likely use the umbrella more and more this year due to shooting open with a lens, a lens is more likely to glare in less than ideal lighting conditions.
I know I'm going to catch heck for this one.......by my umbrella actually matches my chair/stool. [:-]
I don't use the umbrella often, but when you need it........it's a truly awesome tool. I've used it for other peoplemore than I've used it for myself, but it's really a great tool when the sun is glaring, in your eyes, etc. I've also been caught on the course in a downpour more than once and it's handy for that as well.
The umbrella I have folds out to the size of almost a golf umbrella, but packs into a package maybe 13" long, and 2" diameter. It was rather expensive when I bought it, but I've never seen one so compact that folded out so large, and it seems to of quality build as well.......I've gotten 3 years out of it. The pack/stool I use has a pocket on the side of it that works perfect for the umbrella so that the umbrella goes up the side of the stool leg, and I can wrap the bungee on the umbrella around the leg to hold it there secure.
On a side note, the Aurora backpack w/ stool from Lancaster Archery for $29.99 is IMHO the best thing I've seen yet for a stool/pack. I can fit 4-6 drinks in there, a seperate pouch for all sorts of tools, and I've zip tied the plastic arrow tubes to the legs as well so it works perfect, and it's much easier to carry than a traditional stool.