I understand that Drools is Open Source and JESS is not, but how do they compare in other areas like ease of use, performance, level of integration with your code?
When we needed a rules engine, we decided to roll our own, because the available ones were far too complicated for our simple tasks. If you are even remotely experienced with parsing expressions users may put in, this is not very hard to do. In our case, most of the spec is handled by an XSD and only a few of the fields are parsed further.