They work well, but I would only use them on an archery hunt. I've called in a couple bulls into it including one around 350". He just stood there staring at it from about 30 yards away.
So from a setup stand point, it's best to get it setup behind you, so that you can position yourself between it and the incoming bull.
The visual adds a lot to the effectiveness of your calling.
CAUTION: pack it well, as it does have a tendency to want to open up on you, and if it does accidentally, not only might it spook your prey, but it's sure to scare the crap out of you too!!