MPFR itself is pretty easy to use. Unfortunately, if your formula are ill conditioned you can get inaccurate results even using thousands of bits of precision.

MPFR itself is pretty easy to use. Unfortunately, if your formula are ill conditioned you can get inaccurate results even using thousands of bits of precision... so there is no replacement for doing analysis and testing. Perhaps if there were some interval-math equivalent of MPFR you could at least tell when your code might be producing wrong results without extensive mathematical analysis.