I have a viper and agree with everything Matt had to say. The only thing I will add is the Viper is rather bulky on your back but is manageable. The new version that folds down I would have to assume is easier to backpack but I don' t know how stable it is being the sides collaspe down for backpacing. Being the sides arn' t welded it seems like it would almost have to flex when you put your weight out on the corner. The Vice Pres. of Summit posts here often enough so maybe he will shed some more light on the new model.
As far as API, I have used them also and didn' t like the chain, just seems it' s subsustable to break a link. I just feel safer in a Summit.