I'm still on the fence about TPP. It seems like the benefits to the U.S. economy are minimal, though if the deal bolsters lasting relationships with a dozen countries and helps lift millions of people worldwide out of poverty, it could prove very beneficial to the United States in the long-term. I assume that's the way Obama is looking at it.
A few highlights of the TPP:
Lower tariffs generally (good for consumers and export-oriented producers, bad for industries that compete heavily with imports)
Signatories agree to enforce intellectual property restrictions for a longer period of time (good for pharmaceutical industry, bad for consumers of generic drugs. Internet freedom activists have also criticized the TPP for its potential to entrench and expand US copyright law)
It weakens protectionism in the auto industry, like the North American content rules (bad for US auto industry, good for Japan)
It enhances labor mobility so it is easier to move within TPP countries
It includes labor rights clauses, restricting child labor, forced labor, etc. (this will prevent people from challenging labor laws as protectionist measures, and may raise the standards of some developing country signatories slightly)
Ends some forms of agricultural protectionism in Japan and Canada (bad for Canadian dairy farmers and some Japanese farmers, good for consumers and producers elsewhere).
It also contains an environmental chapter, which has received more criticism than the labor rights chapter as far as I've seen.
It will establish an investor-state dispute mechanism (some critics argue this will elevate corporations and enable them to undermine national sovereignty, while defenders point out that most trade agreements have such provisions, and the TPP has included an opt-out for tobacco regulations, after the Australian experience).
The TPP will likely boost GDP among the 12 prospective signatories (US, Canada, Mexico, Vietnam, Brunei, Singapore, Japan, Malaysia, Peru, Chile, Australia and New Zealand). But the gains range widely. Because the US is already a large diversified economy, its projected gains from trade are smaller - only about $78 billion. Vietnam, in contrast, is projected to see an 11% boost to GDP.
Of course, not everybody stands to gain equally. Consumers gain pretty unambiguously from having a greater access to cheaper goods. Workers in some industries stand to lose as they face greater competition abroad, while others stand to gain from access to new markets.
In some countries, however, there are provisions in place to help those displaced by trade treaties. The US has Trade Adjustment Assistance, for instance. And in the Canadian election, Stephen Harper pledged billions to the auto and dairy industry (though it is up to Trudeau as to whether he follows through).
Some of the other implications of the treaty are harder to sort out in a balance sheet type manner. There may be geopolitical implications, as the US strengthens trade ties with many ASEAN countries, plus Japan. The treaty may pose risks to the environment. And certainly for consumers of generic drugs, this is probably not a good treaty.