I am currently not using any mirrored cameras (Digital Single Lens Reflex or DSLR). Instead I am using only mirrorless cameras, which can also be known as DSLM (Digital Single Lens Mirrorless).
Many Photographers prefer the DSLR for the following advantages:
1) The viewfinder is optical rather than electronic. That means no image pixelation, and no image lag.
2) Focusing is by a Phase Detect auto-focusing system, which tends to be very fast.
3) SLR’s (both for film and later digital photography) are well established, and are supported by Canon and Nikon.
There are not really any other significant advantages.
However, Mirrorless cameras, with their electronic viewfinder (EVF), offer their own advantages:
1) Removing the mirror and pentaprism makes the camera body simpler, smaller, and possibly lighter and less expensive.
2) A live histogram can be displayed in the EVF.
3) Use of focus assist technology (such as focus peaking) can be implemented in the EVF. Manual focus with a state-of-the-art mirrorless camera is easier and much more accurate than with a DSLR.
4) The EVF can display what the digitally captured image will look like before it is captured, including its exposure, white balance, and black & white or filter effects.
5) The EVF can perform the same functionality as the Live-View function on a DSLR, but without the restrictions of working from a screen. Handheld, many photographers prefer not to work off the screen, and the EVF can be viewed in bright Sunlight. Also, the shot can be reviewed through the EVF, so you can avoid the motions of chimping. When shooting video, you can monitor through the EVF.
6) When shooting in really dark conditions, the scene can still be viewed in the EVF
7) In many cases you can fully operate the camera controls and access menus (if necessary) without having to take your eye away from the EVF.
8) Because there is no mirror, there is no mirror slap, so that lower handheld shutter speeds can be used.
9) Because focusing is performed on the image sensor, there are no front-focus / back-focus issues.
10) The camera is quieter, and more suitable for working in an unobtrusive way.
Regarding the first DSLR advantage (optical viewfinder), some would consider that the advantages of the EVF already outweigh those of the optical viewfinder. The best EVFs now have such high pixel counts, that individual pixels cannot be distinguished, and lag has also been reduced to the point where it is imperceptible.
Regarding the second DSLR advantage (phase detect auto-focus), most state-of-the-art mirrorless cameras now use “on chip” phase detect as well as the slower (but potentially more accurate) contrast detect auto-focusing. So that advantage is becoming less relevant.
As for the third advantage (support by Canon and Nikon), if you prefer to stick with those brands and you want a camera with a DSLR type design, you pretty much have to stay with DSLR, for the moment (2014). I happen to be one of the photographers who prefer to use what mirrorless has to offer, even if I have to use other brands rather than the reputed “professional” brands. But my prediction is that this will all be changing by 2016. Let’s wait and see.