Not having tried the Leica 18f3.8 I cant really have an opinion on its performance. I like the focal length, have the Distagon 18f4 from Zeiss and like it. Big chunk of glass though. It fits nicely between the 15mm and 21 mm when you need that extra "reach". If the performance is anything close to the 24f3.8 Elmar it would be a phenomenal lens.
Somehow. I wish that they would have made it a 16 mm insteads. Cant justify two 18's and the Distagon is so good anyway that if the Leica lens is better. it probably would be an incremental improvement and the doubling of the price would most likely not justify it.
Nice to see them start doing some new glass though. Somehow I think they are trying to play catch up with Zeiss and Cosina though - except when it come to pricing. Leica is at their best when they are doing cutting edge optics, the 21/1.4, the 24f3.8 and, if your life involves lots of dark places and dingy bars, the 50f0.95.