TDD and unit testing seems to be the big rave at the moment. But it is really that useful compared to other forms of automated testing?

Intuitively I would guess that automated integration testing is way more useful than unit testing. In my experience the most bugs seems to be in the interaction between modules, and not so much the actual (usual limited) logic of each unit. Also regressions often happened because of changing interfaces between modules (and changed pre and post-conditions.)

Am I misunderstanding something, or why are unit testing getting so much focus compared to integration testing? It is simply because it is assumed that integration testing is something you have, and unit testing is the next thing we need to learn to apply as developers?

Or maybe unit testing simply yields the highest gain compared to the complexity of automating it?

What are you experience with automated unit testing, automated integration testing, and automated acceptance testing, and in your experience what has yielded the highest ROI? and why?

If you had to pick just one form of testing to be automated on your next project, which would it be?

Thanks in advance.