[NBJack]

Mocks force you to be aware of how your method/function under testing operates at a low level. At that point your test is now tightly coupled to a specific implementation. YMMV, but I can safely say from working on large codebases across several companies I've seen this get painful at scale when things change, whether it's a simple refactoring or an optimization pass. Mocks also tend to be much, much more complicated on a per test basis.

All that said, I personally prefer the upfront investment in stubs. At scale, it is something readily reusable by other test suites and teams out there.

[cultofthecow]

how come mocks make you tightly coupled with implementation?

Use and depend on abstractions maybe?