I used a poncho last summer. It doesn't protect the legs from rain as much as I thought it would. May be I'm wearing it wrong. I have to sit on the poncho (not directly on the seat, as in there is poncho between your pants and the seat) or else too much wind catches inside. This is kinda awkward and there is a chance of slipping off the seat.
It doesn't cover my arms below the elbow well but I don't have a problem with that. My one came with a hood which I keep below my helmet, so head is completely dry as the torso. In significant rain and duration, your face will be wet and you definitely can't avoid the splash coming from under.