I’ve used the rules engine before and it works very well. The only problem is, it’s somewhat hard to change / update. You would not want business users updating Prod directly. And moving new changes is somewhat extensive… ie it’s a full product deployment.
I’ve used helper .net components for this as well. I believe Enterprise Library has some caching blocks that can be easily leveraged.
If you are in the 500 per second range, rules engine is probably the way to go. But you’ll be looking at a lot of BizTalk Servers to handle they type of load.