Thanks for the comments and links. It's an MM. I will mostly use it for portraits, so center sharpness, colours and bokeh around f/2.0 is what is most important to me. Since I live in Thailand, eBay isn't really an alternative (cost of money transfer, shipment and the risk that some customs officer finds that he wants $100 as well). The Rokinon is available here as Vivitar for $450 (the Zeiss is $370, but should I decide to sell it, I could probably get more for it in Europe). The advantage of the Rokinon is that I can have it in F-mount and use it on the S5 as well, but I already have the Nikkor 85mm f/1.8 that works well with AF, and the Nikkor 105mm f/1.8. The problem with that 85 is that it's useless for manual focus, and the 105 is too front heavy for the GH1.
An advantage with the Zeiss is the compact size. It's small enough to go into my carry everywhere bag mounted on a camera. The Rokinon is a bit larger. I don't know about build quality on the latter, but the Zeiss seems to be built like the proverbial tank.
The adapter is expensive, but again, eBay isn't an alternative, and this is a Fotodiox.