One very successful feedback method I have used with group projects is video feedback. I use a screen recorder such as jing.com to look at the groups assignment, this allows me to verbally point out good portions as well as portions in need of improvement. Students can see and hear the feedback and I can write notes on their document in live time. The response from students has been great they say it is the next best thing to being there in person. I must admit I only do this with group projects as it is fairly time consuming and to do it with each individual student seems overwhelming.