We've just tried Cisco Energywise for a customer who had a large flat Layer 2 network across a Metropolitan area. We enabled Cisco Energywise on every switch across the network for future use with an IPT system, but we hit quite a big problem.
We couldn't quite identify which switch was doing it, but we placed 3 Energywise domains into the network and at least one of the switches then started to broadcast around 3Mbit/s of broadcast traffic onto the network. We had to turn off Energywise on all switches to get the issue to go away!
Switch IOS used was 2960 LAN Lite 52.SE2
Enable with caution - use sniffer traces to check for broadcast traffic through VLANs and check interface statistics to see if your counters are incrementing like mad!