Sury, You are missing a bit. My widest ever was a 21mm on my old x700, and it really showed the difference!
Now days, I sometime use the panoramic feature on my a580, but that's not quite the same as a wide angle. You'll miss the top and bottom, but then again, many times that's just sky and ground, unless you're inside some interesting architecture.
I was planning on getting the Sigma 8-16, but I'm holding off as I may switch mounts since I have EVFs and would like more pixels and better DR that newer sensors have. That's for APS-C. On FF David likes the Sigma 12-24 just about as well. 12mm on APS-c still gives you a tremendously wide 18mm perspective, so it would do you good on the a77 as well.
Of course, if you want a cheap way of looking at the results, Rokinon, etc. have screw on 'fisheye converters'. On wide lenses they distort quite a bit, like a fisheye would, on a bit longer lenses mine doesn't seem to distort, just really wide the perspective.
An a700 and couple of a580s, plus even more lenses.