If you've been using Leicas for 40 years, and are used to the focal lengths, and have quite a lot of lenses in the right focal lengths for film, and shoot film alongside digital, this is not a hard question to answer. I already had 15-18-21-28-35-50-90-135 lenses for 24x36. To get a comparable choice with the M8 I'd have needed to add 12 and 24.
If you shoot ONLY smaller formats than 24x36, and have no investment in existing lenses, and don't need/want fast wide-angles, well, there's much less of a problem.
The M9 looked like a bargain to me, as compared with buying a 24 Summilux for my M8. But I do like the 135/2.8 on the M8.
Cheers,
R.