Archie, I've been shooting my brothers .177 TX, its not convinced me enough yet.
Dave, like I said, PBR for a .22 is roughly 7-35 yards, meaning that you will hit a 25mm target in the cross hairs at these ranges, .177 is roughly 8-45 yards giving you an extra 10 yards or there abouts.
After these ranges you either need to know the distance, or use a range finder for either calibre so it really makes no odds.