A buddy of mine had a wonderduck way back when. Was never overly impressed with what they added to the mix. When we switched to Mojos the difference was astounding.
In low light conditions the mojos are like magnets to ducks. The birds will land virtually on top of them. Put the mojo on the X where you want the ducks to land. I've always experienced the opposite with geese. The few times I've used a mojo on a duck/goose field type set up, it's worked well on decoying ducks but seems to repel geese. You can turn it off if you've got a remote as geese are approaching.
With all due respect to Ken, when I'm duck hunting, I'm a little busy to be fooling around with a jerk string. One hands holding a shotgun, the other is holding a duck call, etc. etc.
The factory installed battery for a mojo lasts at least two full mornings of hunting. The biggest issue I have with the mojos is the added weight. If you're humping in equipment say a 1/4 to a 1/2 mile, the weight of 2 mojos when added to everything else is considerable.
Considerable, but usually well worth it ! JMHO
Steve