My experience 1 month in.
Bit of background on my headphone experience for context - I came from a pair of Sony Ch700n, not Hifi by any means but a great set of headphones with awesome sound and great battery. Prior to that I was using AT MX30's which I gave to my girlfriend. I tried the MX's one day just to remember what they sounded like and that's when I realized how much I missed a good pair of headphones, even though my Sony's are great the MX's just sounded so awesome, so I decided to get myself the HE4XX. All in all, I bought them at $110 USD w/ $20 off and in CAD with shipping, fee's and conversion it came up to around $170 CAD.
So let me actually get into what I think about the HE4XX- plain and simple, they're awesome. Firstly, they're my first pair of open back and I love it. I use them in my home office and I like having the spacial awareness of my surroundings that an open back provide. Secondly they are very comfortable (I wear glasses and have no problem), I've read people saying they're heavy but personally I don't notice it. The adjustment sliders are pretty crap IMO, I liked the snap adjustments on my Sonys and the HE's felt really tight and difficult to adjust. In terms of sound I absolutely love them, I listen to a variety of different genres and they all sounded great (some more then others), personally I felt like anything from Adele and Arctic Monkey's AM album really stood out to me. Rap and hip-hop were also great! I'm not going to tell you how each wave length or decibel of sound actually sounded because I'd be talking out of my ass, but everything sounded great. Also to anyone wondering about open back, yes you can definitely hear the difference. The planar magnetic? Um, not so much, I don't think I'd be able to tell the difference.
I've driven these of my phone (Samsung S10) and use a Fiio EK10 with my desktop and can tell only slight differences, I don't think the EK10 or a Amp/DAC is essential with these however the high/low gain plus bass boost of the EK10's are nice feature. My review is a bit bias because every headset I've used has been an incremental step up from my previous set, so these are the best (and most expensive) headphones I've used. That being said they're awesome. I don't see myself upgrading anytime soon (unless I get a big influx of money) however I do wish I had something that sounded this great for my commute (possibly a good pair of IEM's?).
TL;RD They sound great and very comfortable even with glasses. An Amp/DAC isn't required but its nice to have.
Would recommend to a friend.