Unions oppose the new law, saying it reduces their ability to represent workers. Government and business support the law, saying it will make the state more competitive and attract labor and business.
To put this into perspective, 17.5% of workers in Michigan are members of unions. Roughly the same as in CA.
I am most encouraged by this step, and see the groundswell continuing to grow. Indiana, for example, went Right-to-Work in 2011. The governor's recall in Wisconsin failed. Voters in San Jose and San Diego indicated a clear preference for limiting the power of public employee unions.
Of all the states in the US, Michigan was the Holy Grail of organized labor, with that unique combination of unions + politics that led to its fall.