chiming in late, but thought i would offer some insights as a guy with a new m20 and a 1 5/8" guitar around, too.
is the skinny nut a big deal? no, for the most part. i bounce between it, my 1 11/16" and 1 3/4" guitars without any real issue. is it noticable? at times. it depends what and where you play, and how chubby your fingers are, i guess. if i'm doing cowboy chords i am sometimes ever so slightly inaccurate for a little bit, but i acclimate quickly. if you play up and down the neck and do a lot of single note stuff, you may not even notice.
all in all, i wouldn't seek one out, i'd prefer not to have more but if i liked the guitar, i would try to make room for it. i certainly don't prefer it to a wider nut, but it isn't a hinderance. pretty sure i wouldn't go any narrower than that, though (like a 60s gibson). that can be problematic. tried an old hummingbird in a store and it sounded incredible, but i couldn't focus on anything but the nut width.
as for the new m20s- i have one. i like it. a lot.
i guess you have to understand that it is a very specific guitar. its tiny, has a wide nut and a warm/dry kinda junky sound. i don't think it is a love it or hate it type guitar, but it is very much its own thing. mine has the baggs undersaddle and with the tone rolled all the way back, and run through a preamp, the sound is better than you could reasonably expect from an undersaddle. not the greatest thing ever, but easily the best piezo i've heard. only gets slightly plastic-y on single notes if you hit them hard enough.
as i've noted before, mine had a finish issue that they finally attempted to resolve but i haven't had the time to ship back yet. we'll see how that goes. but the finsh feels wonderful. the neck shape and feel combine to make one of the better necks i've felt, even if the fret edges weren't exactly finished. the nut is wide but not offensively so. roomy, but no struggle involved. i suppose over time certain spots will buff out into a dull shine from use and frequent contact- that's just how it goes with satin. guitar smells great, too.
i have a hoboken and westerly guild, too. they are both pretty idiosyncratic and this is one is no different- it may be the most unique of all of them. it may also be my favorite of the three; certainly the one that gets the most use, if it isn't the best of the three. its such a beguiling little weirdo. once you begin to understand it and delve into what it can do you really start to love it. and since only it does what it does, it won't step on your other guitar's toes.
and since everyone wants to know: did nick drake play one? probably not. or not how we think, anyway. does it sound exactly the same as pink moon, but in your living room. yes. seriously, it is uncanny. despite whatever guitar use with super old strings, recorded on old mics through consoles and onto tape... despite all those variables, that's what the m20 sounds like. right out of the box. you can do other things with it also, but if you want that sound, its right there.
two last things- it is really small and hilariously light.