I have been working with rule based systems for quite a while now.
Having seen Pega and ILOG, BRMS field always interests me. Though I had heard about Drools sometime back, I never got a chance to have a look at it. Recently, I was looking at some articles where comparisons where made between Drools and ILOG and it's quite interesting.
Here are two links to refer to:
Drools Vs Jrules Performance and Future R&D(
I do not want to compare it myself till I use Drools. But here is a comment by Ellen N. Zhao on JBoss Drools vs ILog JRules - an anecdotal story(here):
Commercial software is of inherently lower quality than comparable open-source software, because it is developed by people who are paid for writing software, instead of people who are fascinated about finding the best solution. -- Dirk Farin: Automatic Video Segmentation
I have used ILOG and have found it good. The sheer simplicity in creating Business Object models is amazing and thus making process of writing rules much faster. Migrating rules have been a pain. I need to see Drools.