Well, I made it 2 whole days (though just 1 shopping day) into 2014 without buying a new lens... though this lens was no impulse buy. It's the new Sony FE (full-frame image circle E-mount) Zeiss Sonnar 55/1,8 lens (model name: SEL55F18Z ). This lens has an MTF curve out of this world (stellar light transmittance, essentially - Zeiss have a white paper on the topic). In practice, this means that the lens has extremely high resolution and contrast "wide open" at its maximum aperture (opening). Designing a lens like this is no mean feat - I've never seen a curve like it. The lens outputs an image circle covering the size of a full-frame sensor, like my RX1, but I am currently using the lens on my trusty NEX-7.
After purchasing the lens in Akihabara, we decided to go for a walk and test out the new lens. What better way to test a lens and camera than at night? :)
It's probably a little early for a verdict, but overall, I'm very impressed. The high pixel density, cropped sensor (APS-C size) of the NEX-7 really tests the central zone of any lens.
There is a 50/1.8 lens, with optical stabilisation, already available for Sony E-mount cameras. It's been around a couple of years. It's maybe 1/3rd the price of this lens, but only covers the APS-C image circle, and thus would not be useful on any future full-frame camera. Rather than sink my money into a lens I'll use (happily) until I "go full frame", I decided to spend that little bit more, and "future-proof" myself with a world-class lens.
And I have no regrets.