I have both the M8 and the G9. In their class, both are excellent cameras, and I use them equally. Having said that, the M8 offers superior shot quality assuming you have some decent glass, experience with the camera and like the rangefinder experience. While not a simple P&S per-se, the G9 is a "fun camera" when compared to the M8 and is great for quality (RAW) quick shots with a decent lens and add-ons (shoe for flash, wide angle, even under-water housing). If I have "the shot", or traveling somewhere especially good to shoot (NYC, Paris), it's M8 all the way. But for catching the kids playing, running around town and having a high quality quick shots, the G9 is a wonderful camera. Assuming you've dropped $5 to $6K on the M8, the G9 is an extra $450.00 to have handy and a great value when the M8 is too much metal to carry.
For big prints (A3), I would only recommend the M8, but for 8x10 or smaller, the G9 holds it's own in this contest... but the M8 is the better camera and should be for the delta in price.