Very good - up to a point. In fact, very good indeed - up to a point. I've only flown one (helicopter sim), and I could never confuse it with the real machine. So the situations, emergencies, etc. were always treated as training, rather than the real thing.
It is much easier, and more realistic, to practise various emergencies, as well as more routine training.
As already said, there are savings to be made in costs. Also with convenience. You can guarantee being able to fly whatever route, airport etc. that you want, and not be delayed by weather, other traffic, ATC or anything.
But, when all is said and done, it is a training aid, and being able to fly it cannot guarantee being able to do the same thing in real life.