One doesn't hear much about it because it is negotiated behind closed doors, and it is more secretive than any prior trade negotiations.
I am in general against because of Investor-state dispute settlement (ISDS) clause, which essentially says interests of big multinational corporations override the regulations and laws of each individual nations. Under the ISDS mechanism, foreign corporations can sue a national government in international arbitration over a government's actions if the measures have a negative effect on their profits and economic interests. Various measures, including those for public health, national security, environment, food and drug, responses to economic crises, could be challenged by foreign corporations, regardless of whether the measures are for the public interest. For example, if company A intends to sell a certain product or service and Ontario deems it inappropriate and wants to tax or regulate it, it can't, because those "investors" are above Ontario laws, and Ontario government can be sued and fined by people in New York City and Ontario taxpayers will pick up the tab to compensate the loss of that corporation.
I think the TPP is far more than just a free trade agreement. It goes much further than that in effectively creating a supranational organization which puts large corporation's profit above the citizens of each member state. Noam Chomsky describes it as "the neoliberal project to maximise profit and domination". It will serve the interest of the wealthy class and provides a hidden scheme for big companies to avoid regulation and achieve profit maximization at any cost, and no individual, and no government could get in their way. Bernie Sanders, unsurprisingly, has been a vocal opponent of the TPP.
By the way, Hilary Clinton had always been a staunch supporter of the TPP, saying it is the "golden standards" that will create jobs and improve American families' living condition. Yesterday, she turn 180 degrees and warns against this "unfair bad deal" that imposes a tax on America's working families. Sometimes she changes so fast and you wonder whether it is the same person.
http://m.nzherald.co.nz/business/news/article.cfm?c_id=3&objectid=11674083