Hi Henrik
I got the Super-Elmar 18mm last february together with my M9 and can say that I'm extremely happy with it!
Compared with any other super wideangle lens I own and use (including the great Nikon 14-24mm) it shows very even performance into the corners, is amazingly sharp already at open aperture and especially shows very good resistance against flare (see the attached file).
[attachment=22481:Pano10_47.jpg]
I often use it for stitching very wide panoramas as well.
With the older firmware it showed an often discussed magenta shift into the corners, but with the current firmware (1.116) this is gone.
Together with my Summicron 2/35mm ASPH. and the 2,5/75mm Summarit it makes a fantastic combination for travelling and hiking.
IMHO currently there's nothing better available considering weight, size and file quality (assuming you have a good DNG-workflow).
I think if you're really into extreme wide angle photography and are aware of the limitations of the M9 system (speedwise, rangefinder framing and focussing precision... ) you can't go wrong with it!
You can find more information about Leica lenses on Erwin Putts page
http://www.imx.nl/photoor on
http://diglloyd.com and
http://www.reidreviews.com/reidreviews/ (these two are worthwhile pay sites)
Good luck
Jost