I have a Benchmark DAC1 connected to two computers. The Mac Powerbook uses the USB input, and the PC uses the S/PDIF input. A front-panel switch selects the input.
For output, the DAC1 has a built-in headphone amp with two output jacks. It also has balanced line out and RCA line out. I use the latter to connect to my power amp and speakers.
The left-hand headphone jack has a switch incorporated that mutes the line out. I use that for my headphones, so I'll remember to pull out the plug when using the speakers because the volume needs to be set quite a bit louder then. The DAC1 is right there on the desk, so switching just takes a moment.
I use headphones when I want the best sound, or early in the morning. I use speakers when I need to move around, or am DJing a program and need to use my headset to make announcements. Speakers sound better than that particular headset, but I do have to remember to turn the volume down so I don't get echoes...