I'm going to weigh in here after throwing in some stones here and there.
Mobil makes quality products, in general. Their ATF, gear lubes, greases, and oils are of very high quality.
I think this whole 5w30 thing is a mess, whether true or not. The bottom line is that if the 5w30 is not meeting specs, then Mobil has some issues they need to deal with. However, this is one grade. A common grade, yes, but let's look at the whole picture here. Also, let's give them a chance to really reply and prove one way or another.
I ran M1 5w30 in a GM 3.4 for 6 years, 70k miles, and TONS of harsh conditions. Always started, returned good MPG, and was running good as new when I sold it. No leaks, no nothing.
I have a supply of cheaply purchased Synpower. Fine oil, but from my UOA's and friends, it shears like crazy in the 5w30 grade. Even in my little 2.3 Mazda. Does this bother me? Yes and no. Will it make me stop using it? Not when it's this cheap!
My point is...let's look at Mobil as a whole. Let's give them the benefit of the doubt and hope they give a better response...SOON! If there is a problem, let's just hope they fix it, and quick.
I have to change the oil on the 3 this week. It's cold here, and since I can't find 5w20/30 in M1, I'll probably use M1 10w30...and I'll use it with confidence.