I've been an M9 user since late 2010 and acquired an M240 in February.
The M9 certainly has quirks (which I suppose one could say about Leica gear in general) and the M240 addresses some of these.
I tend to be a fairly demanding user of these cameras, in that I generate a lot of images and frequently in rapid succession.
The M9 has primarily frustrated me with its shallow and slow card write speed. It also doesn't seem to like being pushed to multitask, such as reviewing images while clearing the buffer, which might present lock-ups, which may be card dependent... The M9's reputation is one of being very particular about memory cards. Battery capacity is also in the 300-400 images range, on average, if shooting DNG, which will really depend on your shooting needs and style. For some of my work, I've gone through 4-5 batteries in a day.
The M240 generally addresses these shortcomings to my satisfaction, though I would still like a faster and/or deeper buffer. And I have experienced M240 glitches when multitasking it, too, but not as frequent or terminal as with the M9.
The M240 also generates images that 'feel' more in-line with mainstream brands. Maybe because of the CMOS sensor, it's better at highlight recovery, with a somewhat wider dynamic range. Though I feel its shadow recovery depth is not quite like the M9's, assuming your M9's deep shadows aren't masking random banding. This gets to be quite subjective and results in CCD vs. CMOS debates. At least for me, I don't have much difficulty getting M240 files to look Leica-like, along the lines of the M9. But on the other hand, I also shoot with Canon gear and find it's easier to get the two systems to look similar in post.
From an image resolution perspective, I don't think it's the reason to favor one over the other, though the M240 is somewhat better with higher ISOs. Slightly less color blotchiness, IMO.
To me the M240 feels like a refinement with some useful additions that can be handy, such as the live view/EVF option. It also feels quicker, smoother thanks to the quieter shutter, and more responsive, if you need it to be ready quickly for the next image.
Considering decent used M9s can be found under $4000 US, and that the camera should be serviceable for a number of years (the M-E is the same camera still selling new), it could be a great option, if it matches your shooting style and expectations.
For most of my 'wandering around town' type photos, I could live with the M9 happily, typically shooting at ISO 160 most of the time, sometimes as high as 640. The M240 better suits 'work' applications where I need it to be quick and out of the way, where I don't want to be reminded about a shallow, slow buffer at the wrong time..
