First I am a confirmed M9 user and typically work with 2 bodies doing street and travel shooting. Besides costs ..the M9 has only three weaknesses that bother me and at least on paper ......the Sony Nex 5/7 seem to address these.
1. As good as the M9 sensor is for most of my work ..it is really lacking at ISO above 1000 and even with fast lenses ..not enough . The tests of the Sony show excellent results at 1600 and good results at 3200 .
2. Focus peaking as Jono s post shows opens the Nex up for longer lenses . I would love to use my 135/3.4APO as a 180 FOV . Focusing a 135APO on an M requires painstakingly patient technique . Doing it at dusk approaches futility.
3. Fill flash ... we all get pushed into service for on demand family pictures or have situations where just a little fill flash would make life easier . You can do this with an M ..but its not smooth and easy .
Why not carry a Nex body as a back up that extends the capabilities of your M kit ? I am interested in any insights from the Nex users .