Been a while since I've checked in, but I thought I'd give a relative newbie report on our experiences free-ranging.
We started our first chicks in early 2011, so we're approaching 2 years with them. Since the first ones hit about 6 weeks or so, they've been free ranged - the auto-door locks them in at night and lets them out in the morning. They gradually have increased their range and seem to have set their limits on about a 1.5 acre area around the coop, barn, house, and front pasture.
From our original flock of 30 (two larger groups, plus a 4-some of Australorps):
Losses as chicks to disease/weak birds: 3
Losses to predators: None (except a skunk that took all the eggs from under a broody... he is now buried in the back pasture under a bunch of compost).
Losses to mishaps: 2 (both stuck behind a feed bin on separate occasions.. after #2, I blocked that area).
We've had higher losses with the muscovy ducks. 2 we think simply flew off - they were a bit wild. 1 drake and 1 hen definitely got hit by predators, probably raccoons (I've taken out a good number of them). We lost a few early to sickness, including 2 drowned, but that's not bad for somewhere around 50 ducks (most of which we've sold). Prolific little girls - biggest clutch so far was 18 of 24 hatched - we now steal a few of the eggs when they go broody so they're not leaving some out in the cold.
I figure the ones lost as chicks/ducklings were unrelated to free-ranging, so even if I count all the rest as directly attributable, that's a loss of 4 birds over 2 years, a bit less than 10% attrition, and even if I value them at $20 each, I'll bet you by free-ranging I've saved more than $80 in feed expenses (~5 bags). Plus, my tick problem is almost nonexistent and the flies in the barnyard went down tremendously with the ducks.
The only things I'm planning to change:
1) I'm going to build a small covered area around the coop to get the chickens out more in the rain and snow (the ducks don't seem to mind snow much at all).
2) I'm moving the ducks out of the barn to a separate coop (finishing that right now). They're an unholy mess, and I need their stall when the goats start kidding this March.