How's the Leica DG Summilux 12mm 1.4?

It's quite a specialty item, I would say. For that price, the distortion and CA near the edges is kind of a bummer.

But if you need a lens like that, it's basically the best. To sum it up.

Pros:

• very manageable vignetting (about 1.5 stop wide open, reduces very quickly and is very low already at f/2), especially compared to FF lenses of similar parameters (important for astro)
• fairly evenly sharp across the frame wide open
• fairly well controlled coma (very important for astro)
• can do sunstars with just a bit of stopping down (even at f/2.8 in some cases, if I remember correctly)
• nice, pleasant bokeh
• amazing build quality, it really feels great to handle
• weather resistance
• zero focus breathing (very important for video if any focus change is to be involved)

Cons:

• rather large geometric distortion
• rather pedestrian close focusing capability
• noticeable (but not that big) CA near the edges that's present regardless of aperture
• some extra blue/purple fringing on Olympus bodies that can show up real bad with some types of artificial lights (I use HiTech Formatt's 2A UV filter which seems to solve the problem)
• blue coma and fringing at apertures wider than f/2.8 (very annoying when bright stars are involved)
• doesn't quite live up to expectations when High Res mode is involved (or maybe there's some field curvature that makes things tricky, I don't know), see here

For me, the fringing is probably the single biggest issue I have with this lens. But unfortunately, it's a really common issue with pretty much every fast Micro 4/3 prime.

TBH, I had quite some doubts after splashing so much on this lens and discovering the fringing issues. But then it ended up doing very well for catching the aurora (more in my flickr album ):

And here's a full res sample (unfortunately, under full moon and light polluted sky):

In short, I'm happy with the lens, but I still think it's too expensive and not worth it unless you really, really have a very specific needs. It's one of them special lenses.