Depends on what you are trying to do. If your main goal is automatation of -whatever-, the API may be the way to got. But if it's only about organizing and working some campaigns, the API doesn't really make sense.
The API is not a create once and forget solution. As Google launches new versions several times a year, and only supports the last few versions - you need to update the code on occasion.
Using the API is a create an support solution.
The API is a great way to automate many processes; or tie your internal data to your AdWords account; however, for a small spend - it might not be worth the overall cost of implementation and support.
The API does have some costs associated with it beyond what you would pay a developer: [code.google.com...] however, the trick with the costs is to write efficient code. There are many ways to accomplish the exact same operations with the API, and while one way could be expensive - another could be cheap - hence, it is worthwhile working with someone who is familiar with the API and your goals.