I think that you can make good use of your time by rotating between sets if the gym isn't too busy. I hate losing a set just because someone is sitting at a machine to do all their sets.
I'm going for long and lean not super building, so I do three sets of 15-20 using slightly less than I can actually lift. I do upper body one day, lower the next, but if I'm doing an upper body workout, I tend to do something like:
one set of a "push" exercise (chest press, shoulder lift, fly press, etc)
one set of abs or lunges or such in between
one set of pulls (rows, latpulls, shoulder pulls etc.)
one set of Abs or lunges and then repeat.
That way I do arms, rest while working something else, another arms, rest while working, another arms. Everyonce in a while, I bump up the weight so that I can 'barely get ten out" and that adds some strength. The slow increase in actual weight means I get firm without actually bulking up. Guys who want to bulk up will have a different regime--heavier weights and fewer repeats.
You can sit at a machine and do all your reps with a 1-2 minute break in between depending on what you are doing, but you don't get as much done as if you get up off the machine and work a different part of your body...lift, jump rope, lift, jump rope, lift---or lift, do step ups, lift do step ups etc...