I've been having a look at [WF Rules Engine](http://msdn.microsoft.com/en-us/library/aa480193.aspx) and [NxBRE](http://sourceforge.net/projects/nxbre/) and it seems interesting but I'm unsure of how well it will perform in real life scenarios.

The one I have in mind is something like a fact base with between 10 and 100 million facts and rules like:

Object.Field < 5000 AND Object.Field > 1000 AND IsProperty(Object.Field2) 

I'm using C# and .NET.

**Edit:** I haven't make myself clear (totally my fault) :) I have my own rule evaluation system that uses the RETE algorithm itself ... it is quite fast, it can evaluate the 10 million fact scenario in about 10 seconds... how fast are comercial solutions in comaparison?