The electricity mix has changed dramatically, as discussed by my colleagues from the Haas School recently. The following chart tells the tale:
Notice that the blue line (coal) is diving, while the orange line (natural gas) is picking up the slack. The change seems to be due to the rapid decline in gas prices.
The changing electricity mix has clear benefits in terms of air pollution. According to a recent report from the GAO,
On average, coal-fired units—both older and newer—produced over 90 times as much sulfur dioxide emissions per unit of electricity as natural gas-fired units in 2010. Compared to natural gas-fired units, coal-fired units also produced over twice as much carbon dioxide and over five times as much nitrogen oxides per unit of electricity in 2010.
The CO2 issue is complicated, because long-term impacts on greenhouse gas levels also depend on two other factors, neither of them known with great certainty: (1) the amount of methane leakage during production and transportation, and the extent to which increased use of gas reduces adoption of renewables. The impact on conventional pollutants is clearer, although the public health benefits also require considering the locations of the plants in question and the degree of pollution control. Overall, the shift seems likely to be clearly beneficial, and people in major cities should be starting to see some of the health benefits already.