I've owned the Viper XLS for about a year now and it's nice but has its flaws. It's the older model (not the X4 series) and the foot straps are a pain. The climber portion cable also tends to clang on the way up the tree. Climbing trees hasn't been a problem, although I try to stay away from tulip trees and other slick hardwoods. Nothing grips better than the Tree Lounge I came off of, but nothing is heavier than the Lounge either.

The X4 series solves the problem of the cable clang and adds stirrups instead of foot "bungees". I plan to buy a new one eventually since I've been relatively happy with the old Viper despite these flaws.