Here's a picture I have been meaning to take for a while. It is the Flevobike roof sitting on top of my roof for comparison purposes.
If you look closely you can see the attachments points bolted onto my velo for the Flevobike roof. So as a true comparison the Flevobike roof should move forward couple of cm. Until I put the Flevobike roof on top of mine I didn't quite realise how much longer mine was. The big advantage of the Flevobike roof is that you can take it apart an stow it inside the velomobile. Mine is more effective in keeping the rain out but you either have it on or its at home.
It is the rainy season here now and I have my roof on almost all the time. I much prefer riding without it as you are more connected with what's around you. With it on it feels more car like. The advantage of either roof is that with it on you are more anonymous. Sometimes you just do not want the attention you get and the roof helps with that.