Michigan: ACA Medicaid expansion up to 400K (84% of total eligible)

The official toll as of 9/22 is slightly lower, at 395K...

Healthy Michigan Plan Enrollment Statistics

Beneficiaries with Healthy Michigan Plan Coverage: 395,547
(Includes beneficiaries enrolled in health plans and beneficiaries not required to enroll in a health plan.)

*Statistics as of September 22, 2014

...but there's an even more recent figure given today, as GOP Governor Rick Snyder tries to take credit for the program (note that the article doesn't mention "Obamacare" or "the Affordable Care Act" anywhere in it):

LANSING, Mich. (AP) — Just over 400,000 low-income adults in Michigan have signed up for Healthy Michigan, the state’s Medicaid expansion program.

In addition, the original estimate of the maximum eligible number of MI residents was apparently a bit high (500K); evidently the actual number is slightly lower...meaning that the percentage reached is even better by comparison:

...The state expects enrollment to eventually reach 477,000.