My team have been using JBehave for some time. It uses plain text files to store specifications. Every step (Given, When, Then) is then executed by a certain method which can extract parameters from the step. Scenarios can be indented and well formatted which helps a lot if clients want to verify them.
There are some problems, too. We have switched to Java 6. Sometimes some scenario steps are ignored during execution. It may cause a lot of trouble figuring out where's the bug.
