I won't receive my M10 Monochrom until Monday, but I own a standard M10, and just sold my M246 in anticipation of the new model. So, I can talk about the M10 series in a broader sense, and what I didn't like about the M246.
I found the M246 to offer good low-light performance up to about ISO 10,000- plenty for my purposes, but I won't say no to a higher ceiling. Perhaps more important to me than the high ISO performance is the new minimum ISO of 160 on the M10M (it was 320 on the M246). With the M246, you'll either accumulate a lot of ND filters, or get used to shooting stopped way down when it's sunny.
Much has been made of the size difference between the M10 body and the M 240/246- I don't really mind, personally. For people like myself with bigger noses, the viewfinder on the M246 didn't really cut it- I found that using the 35mm framelines would require my eye to be uncomfortably close to the viewfinder. The eye relief on the M10 is much appreciated, to me. I had the EVF for the M246 (which was not very good), and have used the updated EVF on the M10- everything about the newer EVF is better.
Part of what I loved about the M246 is that the images looked great with little to no work in post. The lack of WiFi for quickly sharing things through my phone was annoying at times, and obviously is remedied in the M10 series.
I don't love the control ergonomics on the M10 series. On the M246, there were more buttons, but they were mostly useful- particularly the ISO button, which you could easily hold while spinning the multifunction wheel. Your options for changing ISO on the M10 are an extremely-fiddly knob that locks in place _like it means it_, or paging through the menu. It's been cold in NY, and I can attest to the knob being basically impossible to unlock and turn with gloves on- it looks cool, but is a step backwards in functionality. The relative lack of buttons on the M10 makes some other basic tasks feel very awkward- since there's no Delete button, to delete a single image, when in playback mode, the Menu button acts as a delete key. I am very surprised that I haven't screwed this up and accidentally deleted something when trying to go to to the menu yet. The M10M also has a touchscreen, which I didn't find especially useful when testing an M10P.
If the M10M is on par with a standard M10, it will feel much snappier in operation than the M246. I didn't realize how slow the M246 was until getting my M10.
My advice, unless you are truly in love with the M10M, is as follows: people are dumping their M246's left and right here and on the FredMiranda forums. Buy a used, clean M246 for around $4k and see how it fits into your workflow. Bored doctors and dentists selling immaculate cameras with a few only a few thousand frames on them are truly a gift to the Leica user open to buying on the secondhand market. You probably won't lose any significant money- up until a few weeks ago, even the old M9M was going for around $3500, so for the time being, I think the M246 has hit a plateau in terms of depreciation. If it's more than a novelty to you and you're using/enjoying it, sell it for roughly what you paid and get an M10M (ideally secondhand again, because they'll be out there soon enough).
For my part, I'm impatient, and was egged on by Leicaphile friends. The M10M is one of very few cameras I've purchased new in my life, and is the most valuable of those purchases, but I know exactly where it fits into my workflow/life after testing the waters with my used M246. Good luck to you.