I think the cracking issue affects a very a small amount of people, but since everyone talks about it here it seems like a lot. The M-100 has 524 reviews on Amazon and of the 524 only 37 are 3 stars or below. Most of the 2 and 1 star reviews are based on people having comfort issues, or not digging the sound signature. Some of the 3 star reviews were people not liking the sound for the price. Nothing about build or defects. If I only count the complaints about cracking issues or whatnot, only 4 out of the 524 negative reviews are about build issues (and one is about the SpeakEasy cable unraveling after a year. My cable did the same, btw). That's a .76% defect rate. It's lower if you discount the cable complaint.
Sure, 524 Amazon reviews isn't a large sample size and very few people take the time to review a product anyway. However, those who have issues, such as cracks, are more likely to complain than those who are satisfied. Since people with defects are more likely to speak up about them it could mean that the defect percentage is even lower than .76%. I've had my M-100 for over a year and it's been abused to a small degree (dropped onto cement on the sidewalk while doing parkour) and they have no cracks.
I have 3 friends who own them as well and one of them has the cracking issue on his Shadow M-100. If all I counted were the 4 of us, our defect percentage would be 25%. Much higher than the larger Amazon population. Head-Fi is somewhere in the middle of that. Out of the hundred or so people who post on this thread, maybe a dozen have had the cracks? What's that? 12%? And that's not counting all the people who read this thing without ever posting, but bought an M-100 because of our discussions. 12% is still high, but it's not representative of M-100 build quality as a whole. Couple that with the 2-year warranty and the cracks almost become a non-issue.