Easy. Use it when you don't care what the type is. For example
for (auto i : some_container) {
...
All I care about here is that i is whatever's in the container.
It's a bit like typedefs.
typedef float Height;
typedef double Weight;
//....
Height h;
Weight w;
Here, I don't care whether h and w are floats or doubles, only that they are whatever type is suitable to express heights and weights.
Or consider
for (auto i = some_container .begin (); ...
Here all I care about is that it's a suitable iterator, supporting operator++(), it's kind of like duck typing in this respect.
Also the type of lambdas can't be spelled, so auto f = []... is good style. The alternative is casting to std::function but that comes with overhead.
I can't really conceive of an "abuse" of auto. The closest I can imagine is depriving yourself of an explicit conversion to some significant type -- but you wouldn't use auto for that, you'd construct an object of the desired type.
If you can remove some redundancy in your code without introducing side effects, then it must be good to do so.
