I'm generally not a fan of out and back but it really all depends. I used to run an out and back route that was 6 miles total on a dead flat and straight bike path and I absolutely hated it, as boring as it gets. My commute to work is similar since it's on a dead straight and flat bike path, rather uninteresting.
However, an out and back up to the top of Mount Diablo is rather fantastic. Hill climbs are better as an out and back than straight and flat.
I will say that loops are, generally speaking, better for safety. What I mean is that typically loops give you multiple options to bail out early and cut down distance if you have a mechanical issue, injure yourself, wind up feeling like crud or any other reason that may make you want to cut a ride short. Out and back rides, generally speaking, go out to a far away point and turn around with no good bail out options. So, once you reach the turnaround point, you're doing the full ride no matter what, barring some ride stopping major issue needing a car to pick you up. You can always turn around early if you know early enough that you need to turn around but you're still going to travel at least the distance you already have.
There's been numerous times I've taken advantage of the loop or figure-8 nature of a ride/run to cut off miles or reduce elevation gain even as far as 4/5's of the way through.