Mason-Dixon obviously had a bad FL poll in 2012, but they were generally pretty good in other states in 2012 as well as in earlier Pres. elections.
They most certainly were not. The following data is all from RCP.
Florida: Their most famous flunk. Showed Romney and Nelson up 6. Obama won by 1 and Nelson won by 13.
Georgia: Produced a (very early) poll showing Romney up 17. Polls by SUSA conducted around the same time showed Romney up 7-8, obviously far more likely in such a polarized state.
Iowa: Was the only pollster to ever show Romney ahead in the entire year of 2011.
Minnesota: Had Obama only up 3 when he won by 8. Funnily enough though, they missed badly as well in the opposite direction on the Senate election, having Klobuchar up by 43 when she won by 35.
Missouri: Actually overestimated Obama by 3 points, but drastically underestimated McCaskill and Nixon, saying they'd only win by 2/6 when they actually won by 15/12.
Montana: Similar to Missouri, overestimated Obama by 4 points, but underestimated Tester and Bullock, saying they'd lose by 4/3 when they actually won by 4/2.
Wisconsin: Only did one poll in mid October, and was more or less at the consensus at the time. Yay, they finally got one!
Their polls in 2008 were generally bad as well. They showed Obama up 4 in PA when he won by 10, McCain up 2 in Ohio when Obama won by 5, Dole up 1 on Hagan when Hagan won by 9, Obama up 4 in Nevada when he won by 13, had Michigan as a tie at one point when it clearly never was, etc. On the other side of the coin, they lowballed McCain by 5/7 points in Arizona and Kentucky. They did have a few good ones, like nailing the results in Georgia, Iowa, and, ironically, Missouri and Florida. But their overall track record in both 2008 and 2012 is extremely poor. In fact, their consistency is almost eerie. With only a few exceptions, you can get the correct result if you add 5-10 points to the Democrat.
A cursory glance at their polls in 2014 and 2010 seem to be pretty good. What I've concluded from this is that Mason Dixon is horrible at polling presidential elections but good at polling midterms.
