We use it for our 80 lbs lab mix, he is a jumper, and can clear a 4' fence without touching it. He is a pretty passive dog but can be aggressive when he wants to.
At first we bought the standard power but that didn't even get his attention, the stubborn dog is a bit stronger, I would have to recommend getting that one at least, it has settings and can be set to a lower level but the regular version doesn't go any higher if needed. I buried the wire around about a 1/4 acre area 3 years ago, only had to repair it once, it's worked pretty good so far.
I don't think it would keep a very aggressive dog or stubborn dog inside always, it doesn't give a shock that they always remember. Ive let it shock me to see how it was it isn't a lot but it is uncomfortable.
I say it is not 100% fool proof because my dog has jumped through it when he really wanted to and I have called him back in right away and he will come back through it since he knows he's in trouble,typically he won't be within 3 ft of the wire.
I think it works good, simple enough to install, but does take some training on the owners part for it to work well.
Hope this helps a little.