I'm a big fan of heroku's scrolls gem[1], it uses a key=value format and is intended to be streamed straight into stdout. We're using it with splunk storm[2] which automatically parses logs in the key=value format and from there you can slice/dice the logs using their query language and graphing. This makes it super easy to spot patterns in the logs where there might be an issue.

[1] https://github.com/asenchi/scrolls [2] https://www.splunkstorm.com/