I recently set up my local wi-fi and the speed difference is HUGE. Instead of it trying to use 3G it uses the local connection which I'm sure takes WAY less power.
Since the Droids apps mainly use the internet I question the people who turn their wi-fi off. It makes no sense. I've left it on and my battery lasts longer.
When I am away from the home it doesn't do anything.
So why not leave it on all the time? It's a battery saver not a drainer.