I used to only go commando when necessity required it (a.k.a. forgot to wash the whites), but now I'm a convert. It's nice letting everything ... you know ... fly free. Plus, it saves time doing laundry, and--now that I'm back at school on the pay machines--saves money.
However, Tuesdays and Thursdays are my PE weightlifting class, so I'm not taking any chances, and it's back into the tightie whities under the gym shorts.
And to answer the common concern: I've never had a problem with catching "myself" in the zipper, even when incredibly drunk. Just pay attention to where your bits and pieces are at--which should be common sense anyway.
Really, the only concern is skidmarks, and as long as ya wipe thoroughly....