Update and warning: Leica M Monochrom Typ246 DNG files are currently incompatible with Apple’s ‘Photos’ App in Mac OS X Yosemite causing the library to crash and potentially lose all existing image files in the Apple Photos library. The bug is in Apple OS X Core Foundation libraries (the rot is literally in the 'core'). Yet more slipshod Apple software engineering—Apple Core Rot, but this time it is a data loss scenario, which is far more egregious than other types of bugs.
Update followup: it turns out that the library is not lost, just that it cannot be opened because it crashes every time. Apple will probably fix the bug at some point.
As a professional photographer, and noting the demise of Apple’s Aperture, it appears that the following approach is the best way to use Apple’s new Photos.app .
First, if you want to keep a copy of Photos around, right click on it and compress it to a zip file. Then remove it in Terminal (the Finder won’t allow it to be deleted, saying “required by OS X”):
sudo rm -rf /Applications/Photos.app
Speaking to professionals, MPG strongly recommends eliminating as a workflow dependency any Apple software product except OS X. Apple has thumbed its nose at both video and photography professionals now, both with hardware and software. Migrate work to Adobe Photoshop CC or Adobe Photoshop Lightroom.
Professional users are now a tiny minority of Apple’s customer base, and thus are at extreme risk of being shrugged off and ignored (well, that’s long since happened). MPG has written on this starting three years ago, and subsequently many times in various ways.
Arne K writes
More serious anger from pro users:
https://preta3d.com/os-x-users-unite/ “OpenCL GPU drivers for OS X are broken and are unable to render but the simplest of scenes”
https://lists.blender.org/pipermail/bf-committers/2015-May/045367.html “going to abandon OS X as a serious 3D/graphics development platform”
MPG: tip of the iceberg. Professional users that relied on Apple for years feel betrayed. The above are just two specific examples, a tiny sliver of the issues.