If you are going to use the poncho with a backpack, the back needs to be longer to cover the pack and the back of your legs. Most backpacker models have a set of snaps or Velcro tabs to allow using the poncho with out without the pack. GoLite has done the job and is the best balance of performance/weight/price IMHO. Equinox makes a backpacker style poncho too.
DriDucks makes a poncho of the same type of material as their jackets and a very light "emergency" model too. Both lack the extra back length for use with a pack.
The Cuben models I have seen aren't much lighter than the GoLite and are much more expensive and you have to deal with the idiosyncrasies of the material. If I were going to spend that kind of money, I would get a Gatewood Cape instead and get a good shelter in the bargain.
The plastic "emergency" ponchos are not cut for use with a backpack. I've considered carrying one for use with a poncho shelter, just in the event that there is a big rainstorm at the same time I want to pitch my shelter.
A belt made from light line with a toggle can help tame a poncho in the wind or brushy trails. It can go up below your pack and ride on top of your waist belt buckle in front. You can adjust the fabric all around to suit the conditions.
I carry a GoLite for my summer day hike backup raingear and emergency shelter. I would need to spend a great deal more to get a minimalist rain jacket at the same 7oz weight, which would not provide as much coverage or ventilation, nor act as a pack cover or shelter.