Why Use Assertions?

The code becomes self-checking.
The code fails at a location closer to the bug.
If you put assertions near the interfaces between modules, it makes it easier to assign blame.
It creates executable documentation about:
- preconditions
- invariants
- postconditions

Assertions In Production Code

Production Compilers
Compiler	Assertion Count
GCC	~9000
LLVM	~13000

Total lines of code for LLVM: ~1,400,000. Ratio for LLVM of Assertions to Code:
1/110

Disabling Assertions

Advantages

Runs faster
Doesn't abort execution on error

Disadvantages

If the Assertion accidentally had side-effects, disabling it will change the behavior
Often better to fail early rather than operate incorrectly

Conclusion

It depends on what you want:
- fail early and alert user to problem
- keep going and accept risk of incorrect output

When To Use Assertions

If you're doing something as critical as the final stages of a rocket landing where aborting could destroy something, it's okay to turn them off.
Otherwise don't

From Udacity's Software Testing Unit 1.