Since I am OP, I'll post a "6 mo" recap of my experience with the Pro 500 PID.
When I watched WLL videos on this machine, it appeared I could pull 5-6 shots back-to-back within 2 minutes and the brew water would be within 1.1F of 200.. An ideal choice..
My reality was been different. For me, I've had to crank my PID from the default of 248 to about 256. There is no flash steam at this point. Some others who have this unit report find they only need a 252-254 PID setting.
The biggest adjustment for me is that the rebound of my machine is what I'd consider 'slow'. I make a 250ml Latte and after this one drink my group was lowered about 5 degrees. I can pull a second shot within 2 minutes, but ideally I should wait between 5-10 minutes to fully rebound. If I'm impatient and do 3 shots in a short time, the rebound time is up to 20 minutes.
This is slow and the cost of a narrow restrictor in the thrermosyphon loop. This is not a good machine if you want to do the occasional party hosting and hope to provide, say, 5-10 drinks within 30 minutes.. It would likely take an hour.
Because rebound is an issue, getting the exact same brew water temp on each shot is challenging.. it will take some more experience on my part to learn how to do this. I think this is one cost of an HX over a DB.
On the good side, I do almost no flush.. In fact, if I do flushing, I'll use up my TS water and affect my recovery time. I don't find filling the water tank an issue. It is an extremely well built machine. I plan to keep it for a year or two.
I firmly believe it is impossible to really know what the brew water is doing on any HX machine without an external thermometer mounted in the group head. This costs about $100 in US and about $200 in Can. so this adds to the cost of the machine.
I am hoping on my next machine, that better brew water temp sensors will exist, but they still keep the aesthetics of the current design.