My first Sony mirrorless was the NEX-3. When Sony first released their NEX system, it had a total of just three lenses. A pancake 16mm F2.8, a standard zoom 18-55mm F3.5-F5.6 and a do-all zoom 18-200mm. I was using the Sony DSLR-A550 at the time, which by DSLR standard was considered a lightweight body, especially when paired with native Sony A-mount lenses. The NEX-3 impressed me in so many ways, despite its many limitations (fixed auto ISO range, an insanely complicated menu structure, zero dial - the 4-way cursor/dial pad was a nightmare to use). It was one of the smallest, lightest body known to man at the time to contain an APS-C sensor along with its more expensive sibling, the NEX-5, and it shoots RAW just like a DSLR at very impressive speeds (up to 7 fps at speed priority). Even though its autofocus system was nothing to write home about, the NEX-3 was considerably faster to focus than any DSLRs attempting to do live-view autofocus, except for Sony's very own APS-C DSLRs at the time. Most of them, like the A550 and A500, had a second sensor to perform PDAF in real time.
When Sony introduced their fixed-mirror bodies, the SLTs, the first impression I got was that Sony somehow managed to crossbreed their DSLRs with their NEX's, and ended up with bodies like the A55 and A33, which were both small and light, yet had the focusing performance that bested even the A550 and A500. By then, many of us have started to wonder if Sony was going to abandon both their traditional DSLRs and their compact mirrorless cameras altogether. It seemed like a logical choice, given the fact that the SLTs inherited nearly all of the major benefits of both systems. Anyone who has ever used, or are still using a Sony fixed-mirror body (the current models being the A77II and A99) would've noticed that the inherent light lost associated with the fixed-mirror system to a degree demands a lot more careful post-processing of the RAW files taken with very high ISOs. Although we haven't seen a DSLR from Sony since the A580 (do correct me if I'm wrong), their NEX bodies continued to evolve, first to the NEX-7, and then the NEX-6, while Sony quietly added lenses upon lenses to the system along the way.
When I first got my hand on the NEX-5T, Sony has just merged their NEX system with their SLT system (in name at least). For a guy who's used nearly almost every DSLR/SLT/NEX body Sony has ever made (including the very bulky A850), the merge was probably a wise decision, even though it confused the heck out of everyone.
So, when Sony went ahead and launched their full-frame mirrorless bodies, the A7 and A7R (the NEX system has existed happily for years in the APS-C realm), I could almost hear the Sony shooters of the world let out one huge collective sigh. It was simply too crazy for Sony to release yet another system, even though it was mechanically and electrically compatible with the NEX optics.
At the moment, my A7R feels very much like my very old NEX-3: it's incredibly light for a full-frame body, shoots impressively high resolution stock images that rivals my Nikon D800, has enough physical controls and customisation to match even the best DSLRs, and the very few native lenses that it comes with are light as feather. The lenses (even the entry level FE 28-70mm F3.5-F5.6) and the body combine to produce RAW files that are as detailed as my D800, but, just like the NEX-3, the A7R suffers from being one of the first borns of many technologies. It's contrast detection AF is slow under less than optimal lighting (although still better than any DSLR trying to perform live-view AF), and you will notice that it cannot attain useful focus at anything below 0 EV, and in most indoor situations where contrast is simply lacking. For someone who shoots primarily still-life and landscape (I can take up to an hour to frame a picture), the A7R is more than adequate, but if you are hoping to use the A7R as a multi-purpose body, it may just fall short of your ultimate expectations.