I had one I bought from Cabelas 4 -5 years back .
I trained my lab that a bell on his collar ment he had to stay in the yard and out of the garden ,the collar without a bell ment he can wander .

But he still will not go close to the road .

I forgot to change battery after a year and he still thinks the collar will work .

I gave the unit and the collar away and just use a collar with the bell .

IMO if your dog is properly trained it will the fence will work .

If your dog is bull headed the fence will not work .
I have a freind that bought one , spent LOTS of hours training his dog (a Mutt ) and it will run over the line knowing the shock will only last a few seconds , then it will be free .

Johnch