Drupal Coder includes the command line utility PHP_CodeSniffer, which parses source code to detect violations of a coding standard.
Actually, there are two utilities in the package, one to detect violations, a second to automatically make changes for those that can be so fixed.
I used the provided Drupal standard and ran it against all of the source code of the Optimizely module.
There are more than a hundred "sniffs" that are checked against. Individually, many are minor and relatively insignificant. A few are downright annoying. In the aggregate, though, I do feel that the resulting code was improved in terms of its readability.
The main benefit I've experienced so far is that I am being nudged into writing doc comments for all functions and classes. At first, I was resistent to doing so because good naming is often sufficient as documentation. As I edited file after file, though, I started to appreciate this kind of commenting as a desirable, consistent practice to adopt.
In one case, writing descriptions about a class and its methods helped me realize that the class was not entirely cohesive and maybe should have been written as two classes instead.
Retroactively applying the Drupal coding standard to the entire module was quite a bit of work. Moving forward, using PHP_CodeSniffer incrementally as a matter of habit should be much, much easier.
Installing Coder Sniffer