I've had mine for several months and I think they are a great value. I purchased them for $169 with an included FiOO E06 amp. You can now source them in the States for $129...wow. That's a lot of can for the money IMO.
The k545's can be had for $199 on amazon so I don't think the comparison is unfair. Some people might be less defensive...
I can't speak to the 545, but I would not call the M50x flat...at all. I think the highs are there without being overwhelming and it's a bit on the warm side with solid bass but please note that I'm listening mostly through my Woo WA7 using both the DAC and amp in the Woo (a tube amp that sounds like a cross between a solid state and a tube...not as warm as a full blown tube amp). They are VERY easily driven and sound pretty darn good straight out of my Mac Air (2012 or 2013 model...can't recall) and my iPhone 5s.
I also have the Bang & Olufsen E6's and at first I thought they were the better can...a bit brighter, I thought better and more forward mid's but less bottom end. But after coming back to the M50x's, I'm surprised how good they really are. Basically a really fun, non-fatiguing and comfortable can that's easy to drive and forgiving. Also, they get as close as anyone could expect to a high end can given the price point. That's my opinion anyhoo...
HS