Thanks for the write-up, Max! I want to clarify something though: how do you handle and account for preemption? As we document online we've oscillated between 5 and 15% preemption rates (on average, varying from zone to zone and day to day) but those are also going to be higher for the largest instances (like highcpu-64). But if you need training longer than our 24-hour limit, or you're getting preempted too much, that's a real drawback (Note: I'm all for using preemptible for development and/or all batch-ey things but only if you're ready for the trade-off).
While we don't support preemptible with GPUs yet, it's mostly because the team wanted to see some usage history. We didn't launch Preemptible until about 18 months after GCE itself went GA, and even then it involved a lot of handwringing over cannibalization and economics. We've looked at it on and off, but the first priority for the team is to get K80s to General Availability.
Again, Disclosure: I work on Google Cloud (and love when people love preemptible).