For two years now I have had a Quarq power meter on my XC bike. Prior to that I had a Stages power meter on the previous bike (sold about 2 years ago when the new bike was coming together). So this isn't a story about what it's like to have a power meter, rather how have I gotten along with the Quarq.
As one might expect with a product owned by a major player in bike parts (SRAM in this case) the Quarq feels like a quality product right down to the battery case being easily accessible and robust (neither of which were true for the Stages). I purchased the cranks and the power meter/spider as separate units and assembled them myself. While SRAM cranks have a robust 3-bolt attachment mechanism (and you can put a Power2Max spider on those arms) the Quarq version relies on an 8-bolt interface to really hold the two pieces together.
The battery lasts a long time. I replaced one last month and that is the first battery in my service records for the bike (making it likely but not definite that this was the first battery). The meter doesn't require much user input - zero is reset autmatically and it is self-calibrating as well; basically get on and pedal.
The absolute numbers are nearly identical to those from the Stages, but I do get a left:right report that the Stages one-sided meter cannot provide. I am slightly right heavy if you're interested; usually 48:52.

Using power is great if you want to keep track of TSS (training stress score) because weekly TSS is the actual figure that should be slowly incremented throughout the training cycle rather than hours or miles or any other proxy for what we really want to measure - fatigue. And TSS is a direct measure of the effort leading to fatigue. At least on the bike.

When I have failed in a race, bonking before the end, I can look at the effort levels that went into hitting that threshold and work on increasing them.

Because cadence is an essential component of power, the Quarq reports cadence to the head unit without any sensor or external magnet for reference. Pretty clever.

Power readings are very stable, independent of bumps or anything that might spike the power momentarily in other meters I have used.

I have an upper chain guide on my bike after having the chain fall off and cost me first place in a race on the older XC bike (it had a narrow-wide chainring that did a pretty good job of keeping the chain on, but a chain guide is insurance against that happening). Quarq makes the spider thicker than normal and the bottom bracket area of my bike is a very busy place with the solid Quarq spider hiding the ISCG tabs and the upper guide behind it. The crank axle bends just enough to close the sub-millimetre gap between the back of the meter and the bolt head holding on the upper guide. There is some light scratching on the Quarq as a result - but it doesn't look anything but cosmetic. Despite years of progress in chain guides (they used to require luck, a drill, a hammer and lots of washers and options to install one on a DH bike - now most bikes fit them quite simply) they still cause issues with other equipment a lot of the time.

I did propose a power-based experiment for an article to Mountain Biking Australia before they closed their doors, but it never got editorial approval and importer backing to get me all the bits I needed to get it running. That's a bit sad because part of my reason for purchase was knowing I could use it for research. Still, I'm always learning about my own legs and what separates a hard race from a very hard race, or an easy ride from a moderate ride. All good things to learn. One of the final articles I wrote for MBA, in fact, was the review of the bike with the power meter on it.

I'd have Zero hesitation (pun intended) putting a DZero on another bike. They're great power meters.