As a former owner of the first Monochrom and a current owner of the Monochrom 246, I think the first one is definitely a capable and satisfying camera. You won't be disappointed.
Having said that, I can also say that the 246 feels like a leap forward -- bigger buffer, larger/better screen, quieter shutter, beautiful viewfinder and more accurate viewfinder framing (particularly for longer lenses like the 90mm), better ergonomics, better quality images at higher ISO's, the ability to shoot video (if one is so inclined), LiveView, the option to use an EVF, multiple metering options, better shadow and highlight rendering, and I'm sure a number of other things that I'm forgetting. Of course, whether one 'needs' those improvements is debatable (and if one even considers them 'improvements'), but for me they all add up to a better camera.
I'm sure I would have been perfectly happy if Leica had not come out with a second version of the Monochrom. But I'm glad they did.