With respect to the poll, I have and will buy M glass (Leica or otherwise) even though I have no current intention to buy a M digital rangefinder and my film rangefinder days are behind me.
I wouldn't bet a stellar IQ increment using expensive M glasses on a Nex except at full aperture.
Stellar? Maybe not, that's a high level to achieve, adjective-wise. Noticeable? Absolutely.
My sample size is very small though... I have but the single E mount kit 18-55mm lens and am not planning on buying any others at this point.
Comparing the one kit lens (do I have a bad copy or are they all similar?) at 25mm, 35mm, 50mm to every M and SLR glass I have, every single manual focus lens new and old outperformed the Sony in terms of resolution and apparent sharpness. The Sony suffers from less flare than some of my decades old glass - a win there for the E team, barely. Sadly the Sony lens shows smearing at the edges and corners yet even the ZM 25 and 35 perform very well in this regard on the 5N.
Actually the sample size being small isn't really a concern - my observations aren't unique, as I've read similar accounts and seen visual evidence presented by others comparing various E mount lenses to current or legacy manual focus primes and zooms.
Back to the purpose of the thread, there was a time where a rangefinder body for me was a good compromise for a do-most-things camera. I especially liked the nature of the finder and how it supported shooting people in their environment. But for landscape and macro my small Contax with a nice Zeiss up front did things the rangefinder could not do, at the near and far end of focal lengths. A Contax 139 with a short lens is quite transportable in fact.
Yet I like rangefinder lenses; the dimensions (width primarily), the relatively short focus travel distance, better depth of field scales typically than my SLR glass. I really like using this glass on the NEX and have recently discovered the joys of a Hawk helicoid adapter on the NEX which drops the minimum focus distance of any M glass by a considerable amount allowing for whole new perspectives. For example a ZM 25 with stock MFD of 1.5 feet becomes something of a macro exploration tool with a MFD of just a few inches. One such adapter expands the utility of every M mount lens in one's bag. This sort of utility is impossible on a rangefinder.
The one thing I miss is getting the true field of view and depth of field characteristics my rangefinder glass can deliver on full frame film or digital sensor. Were Sony or Ricoh to introduce a FF version of their M compatible products I'd be one of the many who would be very interested indeed, although in Ricoh's case they need to up the ante in their EVF first, and probably would if a FF version were to be on the drawing board.
That said, even if crop cameras are the only available host for my M glass aside from paying up for a M9, I don't consider it a waste to put M glass on a NEX or Ricoh GXR or any short flange focal distance camera that can be adapted successfully to use M / LTM glass.