Lawrence Hunter wrote an op-ed for Forbes back in 2012 saying the Fed's (at the time) "$1.3T" of MBSes were "worthless." http://www.forbes.com/sites/lawrencehunter/2012/10/29/are-fe...
My main job was pricing MBSes. The calculation is basically sum of discounted contingent cashflows. Even before the 2008 crisis, back in the mid 90s, I'd often see MBSes with a negative net income. Then I'd get frantic phone calls from upset clients, often accountants or CFOs or brokers, who couldn't understand how a valuation less than zero was even possible, and I'd have to explain how their expected expenditures were going to be greater than their revenues...
I tend to agree with Hunter and Hedges that the Fed's "$1.5T" of MBSes are mostly worthless.
That said, I also understand the reasons why the Fed is making these purchases: increase MBS prices, increase liquidity in the MBS market, and so forth. But the bigger reason still seems to me: bail out TBTF banks.
For the Fed's POV on this subject, they publish FAQs and data on their "MBS Purchase Program" several places:
http://www.federalreserve.gov/newsevents/reform_mbs.htm