It's a horrible sin for people who have to debug other people's code (read: team projects). Normally this team will have style guidelines on how to test, what to test, etc.

My view is that for personal projects and limited scope code you don't have to write any tests. I'm part of an operations team and nothing from the immediate team has test coverage. It's all Perl and shell scripts. Never have I thought "We need more test coverage", and I'm pretty sure only 1-2 people know of TDD. By the time you write the tests (for whatever reason it always takes me forever to get a test harness working with my code and IDE), you could have finished the code, tested it manually, and used it. If anything, I'd go back and write the test coverage before you redesign it, so you ensure functionality is the same before and after.

Let me clarify. Nearly all programs should be tested, and have good test coverage. However, in this particular case your getting analysis paralysis from thinking too much about testing.

To overcome the analysis paralysis, just dont test. For now. Once you've started to get code out there and understand what you really need to test (by seeing what gets broken in production) it should be simpler to understand what you want to test and how to do it.

The most important thing is to ship. If testing is preventing you from shipping, skip the testing. The result of that is that in future, lack of testing will prevent you from shipping. At that point, it will be essential to improve your testing.

In my opinion it is. You should start writing functional tests that test the application from the users perspective. Capybara/Selenium/Cucumber for example are a nice combination for this.

Unit tests are great for catching specific small issues, but you always want to make sure that your users can go through the most important steps in your application. These need to be thoroughly tested.