Owning the PC 363d, HD 598, HD 555, ATH-M50, Bose QC-25, HD 6XX, and a variety of typical gaming headsets, along with a dozen studio-grade microphones, I can say the PC37X's are a pretty good deal if you live within the USA. If you're in Canada or the UK, the main selling point here is that they are offered in limited supply and are mildly unique.: something like the PC360 might be better value however.
The PC 363d and PC37x seem nearly identical though: detachable cord, slightly different color scheme, and no USB sound card. Both have 3.5mm jacks and are 99% visually similar in design; the mic arm seems ever slightly different.
After a few years of use, I found the PC 363d did start to break down. The plastic begins to squeak, the microphone arm starts to come loose at the point of flexion, and the audio cable starts to get kinked where it inserts into the headset. Unlike the HD 555, the plastic hasn't started cracking at least. I really wish Sennheiser used better plastics with some of their products.
I love my PC 363d, but I've primarily switched to a Shure SM7B and my HD 6XX's for gaming. The reality is, the microphone on the PC 363d isn't that great. It's far better than any other gaming headset, but it is still a let down when compared to proper studio microphones or even a broadcast-grade lav mic. Where it excels is in convenience and noise rejection -- it's rarely in the way, it provides consistent volume levels, it's easy to mute, and it doesn't pick up the sound of my keyboard.
When I use it, my gaming friends note that there is no bass in my voice -- I sound shrill and unpleasant compared to even a cheap Snowball mic.
To improve the audio quality of the mic, to get it to something near acceptable, here is what I do though:
-- I use a high quality sound card or an XLR mixer with adapted 5V phantom power.
-- I use a foam windscreen. $15 for a really nice one.
-- vsthost + virtual audio cable + RX5 noise remover + EQ depending on the game I'm playing.
-- position the mic correctly and learn to speak correctly.
When using the USB sound interface provided by Sennheiser, while it drives the headphones just fine, it seems to add a lot of noise-rejecting effects to the mic input. Probably a bass-roll-off too. Using a cheap on-board sound card will create a fairly noisy experience alternatively, but it will perhaps sound more natural. Overall, the USB sound card provided by Sennheiser (363d) still worked better than the mic input on my Intel Motherboard and better than my Android Moto G phone. It's not garbage-- just not great.
If you have a 373d, and find the USB audio device to not be that great either, you can buy a new audio cable that has the 3.5mm jacks instead of the USB plug. So in essence, you can turn the 373d into a 37X for $30, with the addition of some red accents.
Comfort and sound-wise, these are all open-back headphones that are extremely comfortable to wear. They sound great, although if you want a lot of THUMP in your music, you aren't going to get them here. They also do leak sound, so they may not be suitable for work. Great for at home gaming though. They seem on par with the HD 555 and HD 598; easy to drive also. They are not comparable to the HD 6XXs though, although musically they can be just as enjoyable -- better than the HD 6XX if running them off a phone.