I have some reasons for my interest in these cameras that are very particular to me.
I like manual focus. My first AF camera was an EOS 300D in 2004 and I'd been taking photos for well over 20 years before that.
I've never got on particularly well with autofocus, partly because I feel it's letting the camera make decisions for me and I do struggle persuading the AF mechanism to focus on the part of the scene that is MY point of interest. AF is great for snapshots, but I prefer manual for my more 'serious' work.
However, manual focus on any SLR designed for AF is a somewhat unrewarding experience; you have no traditional focusing aids such as a split screen rangefinder. It's possible to buy after-market replacement screens that do have a rangefinder, but they are often dim, won't work with slower lenses and cause metering problems. There is AF confirm, but it's still the camera judging focus, not me. Unlike a lot people, I'm rarely taking photos of fast moving action, so the benefits of AF are largely irrelevant to me. I'd rather take my time, think about what's in the viewfinder and get it right.
I also still shoot film and have a fairly extensive collection of Canon FD bodies and lenses. The A7 offers me the option of taking out one bag of lenses with me to use with both a film and digital body, which happens to be about the same size as my Canon A-1.
I do this that at the moment with a Panasonic G2, but I have to compromise my selection of lenses to take into account the different angles of view on a full frame and 2x crop. The A7 is full-frame, so that problem goes away. I know I'm happy using an EVF for framing and focusing, and the EVF on the A7 ought to be leagues ahead of the G2.
Besides, because of registration distance issues, I simply cannot mount FD lenses on my 5D (or any other SLR system) without using an adapter with an optical adapter (and yes, that would be a lot simpler if I switched completely to Olympus OM or just about anything but FD for 35mm film
)
With an A7, it would be like taking out a couple of bodies, one loaded with monochrome and the other colour film, but adding a third option, digital.
There's a weight and size saving question, too. MF lenses are generally much more compact and often lighter than their modern AF equivalents. The OM primes I have are barely larger than common lenses for Micro 4/3. If I take a 5D, grip and 70-200 f/2.8L IS out with me, that's over 2.5 Kg that won't fit into a shoulder bag.
Now, I appreciate that these are somewhat quirky reasons for thinking that the A7 may be my next camera, but it does look a rather compelling replacement for both my 5D and G2.