If you get involved in a project with 0% test coverage, the first thing you want to do is to improve on that percentage. If the project is very large you will not see exciting things happen in your coverage graphs the first weeks... Better results can be achieved by starting out with Graphical User Interface tests and measuring coverage for that.
We saw really awesome results by measuring coverage with Cucumber and Selenium test. This simplified graph shows the coverage numbers over time for the different approaches:
Another plus you get with this approach is that you can quickly get an idea about the amount of dead code in the project.
Use this only to overcome the time you need to fix your unit test coverage. Unit tests are a better base for refactoring isolated code, help to keep your test round trip fast and require minimal setup.