[jdh]

The initial version of OmniFocus that was released right after iOS 5 seemed to have a negative impact on battery life, but after an update that problem went away. Granted, using any kind of location monitoring is going to reduce your battery life slightly, but the Region Monitoring feature is actually designed to be very power-efficient, and as far as I can tell OmniFocus is using the same feature as the built-in Reminders app.

In my case, I actually found the main culprit of my battery problems was Google Sync (Exchange ActiveSync) when used with Calendars (Mail seems fine). Disabling Google Sync for Calendars and Contacts has resulted in a huge increase in battery life for me (even with the iCloud Calendar and Contact options still on).