The KEF M200s are amazing headphones. They are more similar to W4/W40 in sound signature than to W3 or UM3x.
Their signature is clearly laid back and smooth and I agree, there are clear similarities between them and the Sennheiser HD650.
I compared them with the Weston W40 and to my ears the M200s beat them in every regard but speed. Soundstage is among the largest I have ever heard in IEMs and timbre is fantastic. Resolution is great and there is a naturalness to the tone that always reminds me of good over-ear headphones. I like them more than all higher end universals with BA-drivers including Fitear 334 and Earsonics S-EM6, ASG or Stagedivers. Some of them might still have the upper hand technically (resolution, speed) but all of them sound kind of dry and grainy compaired to the KEF (just my opinion). A close relative to the M200 are the Sennheiser IE800s in terms of general sound when used with EQ. The IE800s sound a bit punchier, similar resolution. Without EQ the IE800s sound brighter. It is mostly a matter of preference between the two.
The M200s are great if you get a good fit. Stock tips are useless for many people.
Btw. I know from a KEF distributor/retailer in Asia that they have received many complaints from customers regarding fit. This guy himself told me that the KEF simply don't fit him and therefore he hesitates selling them to (otherwise mostly satisfied) KEF customers. Asian ears might just be too small for them. The distributor forwarded the complaints to the engineers and it is very likely that we will see a redesigned successor of the KEF M200 in 2014.