Hello,
We use in our systems, jbpm because of drools. We had worked with drools in the past and it is a strong and proven platform. So not only because of the ability to add rules, but due to the fact that drools architecture is in the core of jbpm, hence they share sessions, objects, rules, facts etc. all this makes it quite powerfull.
In addition i also like the way that human tasks and custom services are handled and created. Everything seems straightforward, modular and flexible.
However, i must admit that Activiti is really powerfull as well and we keep evaluating both as releases come out. The toolset of Activiti is pretty powerful so due to the fact that both platforms use the bpmn2 standard we try to combine both technologies to some extend.