Yes, Esper is fantastic. Hard to believe that it's free.
The performance is phenomenal... I'm running 500+ continuous queries at 1-2% CPU load. Coral8 crashed around 50.
EQL has far more functionality than all of the commercial event stream processors I've used (Apama and Coral8). The ability to group events into separate (i.e., no fixed-length), different length windows WITH aggregation is next to impossible in Apama or Coral8.
The only downside is limited XML support and no native scalability or high availability features.