Here's a plot of my heart rate over the 43 minute workout described above. You can make out where each climb started, and kind of see the pattern that repeats for 10 laps. I don't know what happened the last three laps. I wasn't aware of it at the time, but obviously I stepped up the intensity.
I'm no exercise scientist, but I believe this kind of thing is good for us. I mean, it will either make me collapse dead or eventually get a really strong heart and lungs.
I just need to talk myself into doing this once a week, an hour or so of tempo twice a week, and a long, moderate effort (3+ hour ride) about once a week, plus maybe some strength training. I'd be one healthy cat!