A Digital Single Lens Reflex  or DSLR has been the holy grail of quality for digital cameras for over a decade.  Kodak started taking Nikon film SLR's back in the 1980's and stuffing them with electronics for digital capture, ever since camera companies have been working at making them smaller and better.  I used to have a 1997 Kodak DCS 620x.  a "high speed" digital SLR based on a Nikon F5 body for shooting sports.  It sported a high frame rate of 4FPS, 2.0 Megapixel capture, and a Super high ISO of 6400.  :-)  Now just about any high end point and shoot can beat most if not all of those specifications.  What then cost more than a car, today costs as much as a tank of Gas for your SUV.

Digital however is NOT film, it poses its own technical troubles, and its own abilities.  The SLR form factor was convent not because it is or was the best, but rather because it was convent as the old technology was fairly well developed and just needed to be adapted for the new recording media.  Just because that is what we now have, does not mean that it is the best form that a digital camera should take.  There have been some interesting developments in digital cameras in the last few years.  Olympus with their Micro 4/3 camera formats have done away with their reflex mirror.  Sony going another step further and keeping the mirror in order to deal with the shortcomings of the micro 4/3 format.  The mirror allowing the advanced phase detect auto focus system to be kept which is a huge advancement over the contrast detect system of other mirror less systems.  Nikon's advanced system adds some of those phase detect sensors right along side the image capture sensors on its image capture sensor.   This seems to be the best idea.  the AF system is all contained on one sensor, Not among several which can go out of precise calibration from one another.  Add to phase detect Auto Focus a system that can identify faces and you can precisely focus on people.   Add eye detect AF, and where you look at the frame is where the camera will focus.

I think that the SLR has run its course.  No longer do we need a mirror that flips up and down to see where the camera is focusing.  We have amazing Speed now in our digital cameras as well as unprecedented sensitivity at the sensor, unparalleled resolution, computing power and small bright and detailed displays.  We have shrunk the system components to the level that they are very compact, and very energy efficient.     Today's digital cameras are an amazing technical engineering achievement, and unthinkable from just 10 years ago.  In fact the mirror is now a problem, the required tolerances are beyond what we can accurately repeat.  The weight and complexity of the mirror system is holding back the size and weight and efficiency of the new systems.   The industry continues to change, and so will the tools we use, but rest assured that the mirror has its days numbered.

Let me list some advantages:

Digital SLR's

Auto focus speed (Phase detect), Bright viewfinder with zero lag, and stunning clarity, accurate viewfinder, accurate metering for both flash and available light, Larger camera bodies to accommodate the mirror and sensors.

Mirrorless

Auto focus accuracy (using the imaging sensor for AF has its advantages),  Auto focus available on the entire frame, Viewfinder 100% accuracy,  viewfinder able to show the exposure that WILL be taken, with live histogram,  Viewfinder brightness NOT dependent on available light OR aperture of the lens.  Camera MUCH smaller,  Form not dependent on a mirror box or viewfinder,  Lens design are less compromised due to limitations due to  mirror distance.  Frame Rate higher without mirror.

I have both in my bag, what will I be using in 5 years, only time will tell