My 6 - 7 day trip became a 4 - 5 day trip when trail I expected via an old USGS map wasn't there ... long road walk to cell reception. Anyway, I was "fortunate" to have rain a couple of the four nights I was out, and the definite threat of rain all 4 nights so I got some experience.
All four nights I put the tarp up diagonally, i.e., one long diagonal parallel to the tent, the other two corners staked out to the sides. When I did this with the Golite tarp and my HH backpacker a-sym hammock, I found that the lateral (against wind) protection was skimpy on the right side of the head-end and the left side of the foot-end. Fortunatly, I had little or no wind, just rain falling straight down.
My hammock got somewhat damp the first night anyway; I'm not sure how (I'm not incontinent, honest ). Might have been that I failed to put up drip lines. Might have been that I snugged the poncho down too well around the hammock, for better side protection. The result of that was that I felt that if I moved much or sat up that I'd put the hammock body or netting in contact with a poncho that had condensation moisture on the underside.
The last night I followed the advice someone gave here previously I think and threw a line over a limb of one of the supporting trees and pulled up the hood of the poncho. This was much much (much) nicer. Since I was in trees and had little wind, I went ahead and held up the sides with my trekking poles too, so that the poncho covered the hammock (barely on the right head-end and left food-end, but did cover it) and I got no moisture at all in or on the hammock as a result. Obviously it would have been a different issue with high wind.
The ideal way to attach the poncho is an issue too. I tried one night with a completely separate line but it made it a lot more time consuming to get everything setup that way. If it was raining when I setup, I guess I'd still do it that way, put the poncho/tarp up higher than needed, put up the hammock, then readjust the poncho.
When it wasn't raining, however, I found it easier to put up the hammock first and then attach the poncho to the tree straps at both ends.
Bottom line for me is that I'm mixed. I liked using this poncho a lot walking on the trails, going through brush (with a little shock-cord belt to keep the fabric in). It worked fine as a hammock cover in the conditions I encountered, but I'm concerned about wind. At some point I'll try to put it over the hammock not on the diagonal, but just with the length of the poncho along the hammock. Hopefully that will handle the wind situation. It will require two more stakes, but more importantly, I'm not sure off hand if it will cover the very ends of the hammock well enough. TBD.