Dislcaimer: This review was written from the perspective of a beginner audiophile with admittedly limited equipment. Take this and all subjective reviews of audio products with a grain(s) of salt, for the good of your wallet.
TL;DR - They're worth the money, fantastic headphones.
The HD 6XX is, by all accounts, a blue colored HD 650, with a 3.5mm terminated 6ft cable. For $200, which is (at time of writing) $150+ off what an HD 650 will set you back from many retailers. This in an of itself is a good deal, but let's talk about the sound...
I've used my pair a fair bit, on a couple of different amps and sources. Everything from a Google Pixel, straight out of the headphone jack, so a Shanling M0, a Fiio Q1 Mk-II, up on to the SMSL M6 and Aune X7s. No matter what source it's on, it sounds exceptional from just about anything around it's price point. But more than anything else, it does sound better when fed from different sources. Class A from the X7s sounds great, but the M6 is still fine. The Q1 is great for portable, but single ended, it doesn't output much more than the Shanling M0. The 6XX fails to disappoint. Bass feels deep and it slaps your head. There's some soundstage, but it's still a pretty intimate headphone, and the highs don't offend. I don't know if it truly sounds "veiled", but there seems to be less energy in the highs than the HE 4XX, the HD 58X, and it's much much less trebbly than the HE 350.
They're comfortable. They do have a pretty hefty clamp on my average size head, but this goes away the more they're worn. Build is on par (i.e. identical) to the other's in the line, as it should be. No tools are required for disassembly, which is fantastic for quick repairs, cleanings, and parts swapping.
Comparitively to the HD 58X and the HD 660?
The vocals are a step forward, to my ears. The highs have less energy in them. These headphones are good at resolving detail. There was a part in John Mayer's Waiting on the World to Change, where a guitar in the background plays like, out of phase with the rest of the music, and it was borderline distracting on these headphones. I personally find the 6XX to have better detail retrieval and a more engaging sound than the 660 (however, I did, admittedly, not listen to those much). The 58X though? While the sound is similar to the 660, and to my ears, less resolving, I find myself reaching for the 58X more often, taking the 58X with me to work, while the 6XX sit in their presentation box... These are a step above every other headphone that I've heard from Massdrop.
The only headphones I tend to have on my head more often than these, or the 58X, are the T50RP/T60RP, for different reasons.
These sound fantastic. While the sound signature isn't for everyone, and I'll admit, there's times I don't want to listen to these, they're still fantastic all rounders, and while different than the 58X, I'm not sure they're truly better. For the price, they're amazing headphones and will always get a recommendation from me. If you don't have a pair yet, you owe it to yourself to listen to this piece of audiophile history. Worst comes to worst? Sell them when the drop isn't active! But seriously. There isn't a better pair of headphones for the price, in my opinion.