How long you've been using this system? Has it been tested by a decently-sized period where you had a large workload?

I started using the system about 6 months ago, but I used it infrequently for about half the time; so, it's still fairly new. I've used it for some fairly heavy workloads. I'm going to try to use it to maintain a self-directed ≥8hr/day workload over the summer, so that'll probably be the big stress test.

Intuitively, needing to always be in a block feels like a considerable amount of bloat in the system -- maintenance work that you're imposing on yourself without sufficiently large gains.

They don't take that long to set up - maybe 15-45 seconds. They do cost me some time, but it's more than repaid in increased productivity.

If I'm trying to learn something, I don't want to take a break in 25 minutes when I've already loaded stuff into my brain. Maybe more variable length work blocks would be better here? (Or maybe you already use them to a greater extent that I thought when reading your post?)

What actually happens in practice is that if I'm sufficiently engaged in an activity I either work straight through the breaks or I forget about the timer altogether. (This seems to happen a lot when I'm programming.) Since this tends to be my most productive time, I'm probably going to come up with some rule change to legitimize this; perhaps ad-hoc conversion of any productive block to indefinite.