Studies by Scotty Cameron and Titleist show that the ball sits in a slight depression on the green and therefore it is advantageous to have positive loft to get the ball out of the depression. Guerin has technology that they say (and video seems to show) that will produce some top spin even with positive loft. I have not seen any studies that indicate that this makes a difference in putting success. It would be interesting to do tests on Guerin vs 'normal' putters using Pelz' putting robot.
BTW - Robotic putting tests have a basic weakness in apllication to people because as Pelz has shown non-pros do not make consistent contact on the center of percussion. As a result the best putter might well be one with a higher MOI even if it does not produce overspin and overspin is advantageous. For that reason I do not use a Guerin even though I find the technology interesting but instead use a very high MOI putter from Golfworks.