The Sony a7 series of cameras has been wildly popular in the last few years and has certainly stirred the mirrorless versus DSLR debate, along with other popular cameras like the Fujifilm X-T1. I'm not here to reignite that debate, at least not intentionally. Most cameras these days, mirrorless or DSLR, are incredibly advanced and capable machines that will give excellent images in almost any situation when used correctly. Of course, some excel where others fall short, but nonetheless, camera performance is at a remarkably high level.
I've always been intrigued by mirrorless offerings; features like WYSIWYG (what you see is what you get) viewfinders and the beautiful colors of Sony and Fuji sensors were quite tempting, but I never sold off my Canon kit and took the plunge for two reasons: lens availability and AF speed. Sony's lens lineup hasn't been particularly well filled out for this system and while most standard lenses have begun to appear, specialty portrait lenses, super telephoto, and higher-end low light zooms have been lacking. This really limits the native abilities of the system. On the other hand, Fuji has been releasing lenses at a blistering rate, but unfortunately, the APS-C sensor takes it out of the running for me. That's not to say it's not a great sensor, but as someone whose work frequently requires high four-digit and occasional five-digit ISOs, I just couldn't sacrifice a full-frame sensor (though I will say the high ISO performance of the X-T1 is highly impressive for the sensor size).