I just (a couple of months ago) finished my first tarp. I didn't really want to do it because of the horror stories I heard from people about sewing the thing. However, I didn't really run into any problems. I made an 8'x10' rectangle tarp, and it works great. It isn't anything to look at for sure, but it works. I just cut two 10' lengths of sil and sewed them together so the tarp would be 8' wide. After I sewed the pieces of fabric together, I reinforced the corners and put extra tieouts in the center of each edge. Pretty simple and no cussing or hair pulling. But because I took the trouble to make the tarp, I feel like I have to take it every time I go now. Oh well, at least its light.
To answer the questions though, I always have my tarp just kind of an emergancy. It is't heavy at all, and there's some security in knowing that what ever weather comes, I'll be ready. I'm sure now that I've read this thread, I will be taking a trip or two without it, so I can take some pics of the stars from my hammock right before I go to sleep.