I usually think it's funny when it happens. The majority of the time, I only see the 'corner of the eye sideways glance'. Only a few guys have ever made a direct comment about my cock. Usually, I figure any attempt at "chit chat" at a urinal from a total stranger has other purposes in mind... especially if that "chit chat" is accompanied by the I'm-looking-down-but-not-actually-looking-at-your-cock-oh-wait-yes-I-am-double-take.
The direct comments I've got have been pretty unimaginative and I guess they were designed to flatter me. The most unexpected was from this older guy -- maybe 70s -- when I was in my mid-20s. I was in a hotel at a conference and he came up as I was taking a piss and he stood there for a second, looked down at my cock, looked up at me and said: "I used to have a dick to be proud of, too". It was the way he said it that left me wondering if I should say "thank you" or "I'm sorry". I didn't look down as he fished out his dick because I didn't want to know. I sort of smiled and nodded and got the hell out of there.
The best comment I ever got was from another military officer that I was stationed with. All of the women I knew that worked with the two of us thought he was God's gift and I had seen his cock before and it was nice. I used to be kind of jealous of him because he was a few years younger, had a way more chiseled body than I did and was one of those effortlessly adorable rogue types that everybody just liked. Anyway, we had been drinking at a function one night and ended up at the urinals about the same time and he leaned way over (like his head was practically next to my pec) and deliberately stared at my cock while we were both pissing. I said, "what are you looking at?" and he answered, "a fucking work of art, man".
Nothing more was ever said... but the implication of possibilities always lurked in the back of my mind after that.