Great choice, I love my 12mm.
What made you chance from the d600 to the OMD?
Bulk and size mainly, don't get my wrong the image Quality from the D600 is stunning but whats the point if it gets left in the house?
I had an EM5 kit last year which I really enjoyed with a 7-14, 17, 25, 45 and 100-300 setup which fitted easily in my small day to day bag, which meant I could take it everywhere.
The other reason is for travel, I took a D7000 to Paris this year and because I'm a huge prime fan I found myself swapping from 24mm to 35mm all the time, I took the D600 with a 14, 24, Sigma 35 and 85mm to Italy last month and while I didn't mind swapping lenses (I used the Sigma 35mm mostly) it was a pain in the neck and made me realise I wanted something smaller.
I did consider just buying the 24-85 Kit Zoom for the D600 but having tried it it wasn't outstanding optically and it didn't really reduce size for me.
The EM5 is the logical place for me to come back to, excellent prime lenses (12, 17, 25 & 45 for now!) and I know the IQ is very good too, I may be losing on High ISO but I prefer to rely on fast lenses.
Hand on heart if the AF was better I'd have bought a Fuji X-Pro 1 before the EM5 but I realised that I need good AF and while it might not be as good as my D600 (although the AF on that wasn't stellar) it will do me for most things.
I've half got an eye on the EM1 as well but want to see how I get on with the EM5 again first!