Paul Hogarth has this detailed analysis of how California’s top-two system has worked in both 2012 and 2014. It appears at Daily Kos, and the impetus for the analysis was the New York Times op-ed by U.S. Senator Charles Schumer that ran on Tuesday, July 22.
Hogarth does not mention these factual errors in Senator Schumer’s piece: (1) Schumer refers to the November election as a “runoff”, but federal law since 1872 has required that the election itself (for Congress) be in November, and if states want congressional runoffs, they must be later than that; (2) Schumer says most states don’t let independent voters vote in Democratic and Republican primaries, but only 15 states don’t let independents vote in Democratic congressional primaries, and only 19 states don’t let them vote in Republican congressional primaries. Schumer also deplores the 2014 Republican primary in Virginia than unseated Congressman Eric Cantor, but Schumer doesn’t seem to know that Virginia has open primaries.