OTF (Orangetheory Fitness) is basically interval training with a heart rate monitor. They have "strength" days and "power" days and "endurance" days. Depending on how many people show up, there are either two or three groups that rotate. I prefer two groups, because this divides us into a treadmill group and a weights / rowing machine group. (Which means fewer switches than treadmill, rowers, and weight groups.)
My preference, since one of my top goals is to gain and maintain strength, is to go heavier on the weights than is always called for by the WOD (Workout of the Day). This causes me to pause a few extra seconds between sets. For example, today I used a forty pound dumbbell for snatches. (That's not going to seem like much to a powerlifter, I know. But when you do a lot of reps and superset it with squad jumps, alligators, and whatever other nutty stuff the program calls for, it creates oxygen debt and vast pools of lactic acid.)
Other people in the class are there for the cardio super-burn. They select weights appropriate for them and run on the treadmill at their own speed (much faster than I run, nine times of ten). On my left might be a marathon runner cruising along at ten miles an hour, while the person on my right is just starting out at a walk.
We all get a great workout.
As is often the case, just showing up turns into the best workout of the week. Go figure, right?