I have owned the QM67 double boiler since 2015, and it has been my daily driver for over 5 years.
Here are my personal impressions and experience.

1. Tight brew temp control
2. Steam boiler too small and way underpowered (but OK for my 2 daily 5 oz. Capps). This is the main reason I would not buy the machine again. This issue is compounded by the fact that the PID gives priority to the Brew boiler (both can not be on at the same time due to the machine's wattage rating), and it can take FOREVER for the steam boiler to recover. If I were to design a double boiler with PID, I would use a pressurestat on the steam boiler NOT a PID.
3. Solidly built but messy wiring on inside with very little room to work (like all Italian machines I've owned). Silver painted styrofoam-type boiler jackets are starting to crumble. I've noticed QM has since switched to the neater black velcroed jackets.
4. Noisy vibratory pump - I switched out the 52-watt EAX5 ULKA pump for the 42-watt EX5. Besides being quieter, it made the world of difference to my extraction. Slower ramp time with longer infusion time and no channeling. I was also able to dial down the brew pressure to get very even extraction.
5. Shot timer was a novelty at first, but now it is just an irritant, as it blocks the PID temp display during the shot. Also the alternating PID display between brew and steam boiler temps is irritating. I do not need to see the steam boiler temp - there is a steam pressure gauge which is much more pertinent.
6. Upgrades: I changed the standard mushroom/gicleur top of the E61 (the TEA coating had pretty much worn off from scale over the years, exposing the brass) to the new style all-stainless "large-cap" style mushroom/gicleur. Much easier to clean (and looks better in my opinion)
I also changed out the rotary-style steam/water knobs for the toggle kind. Just a personal preference.
7. Maintenance - just the usual regular backflushing/cleaning/descaling. I take apart the E61 group every 6 months, and replace gaskets and valves as necessary.
8. Issues - Only things to cause the machine to fail in the first 4 years was 1 pump and 1 expansion valve seat mushroom.

One major squawk just popped up after 5+ years. When starting up from cold, brew boiler probe was reading 55F less than steam boiler probe. The GH temp was too low and unstable. Increasing the PID offset to its max (45F) was not enough, and even setting a higher brew temp target resulted in wild temp swings when measured with a Scace. I was told by CC techs (who have always been great) to replace the brew boiler temp probe ($65), which I just did. No joy. I just wish they had asked me to test the resistance before I bought a new probe. It tested 55.5K Ohms across the one I removed which would make it a themistor. Apparently the brew boiler and steam boiler have different probes.

So I guess my next step is to swap out the $150 PID. I'm thinking of wiring the machine for 20A (may have to increase the gauge of the power cord and/or some internal wiring - though a cursory look tells me that they may be sufficient). I think there is a way to program the PID to kill brew boiler priority, and allow both boilers to fire the the same time. The M58 PID (which I think uses the same firmware) has a parameter setting that moves it from 15A to 20A mode - this same parameter is also on the QM67 PID, but the manual only allows the 15A value.

Thanks for this post. I know it's a little old. I only seem to get lurking every few months. I've had my QM67 for just over two years so I'm finding your experiences useful. I would be interested to know if replacing the PID solved your problem with the heating.

I loved the machine at first but it replaced an old Estro single boiler, so no real comparison. I soon found the same issue you did with the steam boiler. Worked fine for my cortados, less so for my wife's milk heavy lattes. I ended up steaming the milk first for her lattes but I still had to catch the steam boiler right at the end of it's ramp up if I wanted to do it in one go. Not really an issue for regular sized cappuccinos and other drinks with less milk. And don't get me wrong, I still loved the machine but might not have got another one.

After the first year (where I didn't really want to mess with anything) I finally read up on the PID settings and switched it so that the steam boiler got priority over the brew boiler. Not as good for an espresso, but good for a latte with lots of milk. Happy wife, happy life.

Three or four months ago I did some more digging and found an old post on here about switching it to 20A mode so both boilers could heat at once. I emailed Chris' Coffee to ask about it but the person I corresponded with said the 67 was 15A only. I went ahead and switched the F02 setting in the PID and sure enough it now heats both boilers at the same time. I was going to swap out the power cord for a heavier one with a proper 20A plug, but in the end I went ahead like it is. It seems fine. The only time both boilers come on together for any length of time is when I first switch the machine on and the cord is not getting warm (Disclaimer: I am a mechanical engineer, not an electrician or electrical engineer. I did not do any calculations!).

The only issue I've had so far that needed actual repair was replacement of the vacuum breaker/overpressure combo valve for the steam boiler. It was done under warranty when the machine was about 1.5 years old.

At one point in the first year the expansion valve was squealing quite a lot, sometimes for 15 or 20 seconds while extracting a shot. The dealer sent me another valve but it made noise too, although not as loud or for as long. I ended up reshaping the rubber seat on the end of the plunger in the expansion valve using a drill and sandpaper, which solved the problem.

So all in all, with both boilers able to heat at once I find I am much happier with the machine again. I got a fairly good deal on it when I bought it and I would probably buy it again if I got that kind of deal, especially with being able to run it in 20A mode.