What effect does tasker/locale have on battery life?

For Locale, Both GPS and Wifi triggers can drain the battery more quickly.

Locale's built-in location condition can consider cell towers without GPS if you set the diameter of the location condition to about 2km.

There is also a Locale condition plugin, "Location (energy-saving)", which uses cell towers to determine location. It takes advantage of the fact that the phone is already listening for towers. The biggest potential drawback for using the energy-saving location is when you are in an area dense with cell towers, like a major urban downtown with lots of tall buildings (especially if you work higher up where lots of towers are visible to the phone).

I am a long-time user of Locale and a fairly new user of Tasker. I find both to be very helpful tools in managing my device automatically.

I use Locale for all GPS-related tasks. Vibrate when at work between work hours. Silent when at home between sleeping hours. When I first installed Tasker I ported over these profiles and found that my battery life was decreased dramatically no matter how I tweaked the polling settings.

Tasker is more of a action/response application in my setup. When placed in the Car Dock, Tasker autokills the Car Home application, enables Bluetooth, and launches Google Navigation. When headphones are plugged in launch the Cubed music player. Disable vibration on all notifications when the computer USB cable is plugged in.

I don't mind running both or having paid for both but I was disappointed in that I couldn't accomplish everything in Tasker. The dual setup gives me at least 30% more battery life than running Tasker alone.

If you do choose Tasker, be sure to experiment with the polling frequency of GPS and network-based location. That also helps vary the battery usage dramatically.

If you trigger events by GPS co-ordinates (and possibly wifi), then they'll drain your battery significantly, like any other GPS app would do.

If you trigger events by date/time, etc. then you'll experience minimal battery reduction. Often the benefits of these apps weigh beyond any downside, i.e. turning your brightness down or screen timeout down when your battery is getting low.