I went to the Forest Cafe on my first day in Edinburgh. I was tired from travelling, cold, and wet. I ordered tea and while I drank it they let me use their outlet to charge my camera batteries. I had to wait until noon to order my meal (before that simpler options are available but I wanted a hot, hearty meal!) but once it arrived it was great. I ordered the soup and salad combo and they offered tortilla chips in place of bread as I'm gluten intolerant. The soup was thick and steaming hot and my plate had two salads--a green salad with a simple vinagrette dressing and a bean-cashew-sesame seed salad. The soup and salads were all rather simple, none bursting in unique flavour, but they were cheap, hot, and filling. At the time, that was all I cared about.
I really admire as well that this cafe is completely volunteer run. The service was efficient and fairly friendly and I wish I could have attended one of their music or movie nights.
Also: they have both soy and dairy milk on their counter for tea/coffee. Make sure you choose the right one!