Although solar PV is now able to compete on its own (ie without subsidies) in many places, the U.S. federal investment tax credit (ITC) has been very important for helping to speed up the adoption of solar PV technology. Since 2005, the ITC has provided a tax credit of 30% of the value of solar projects.
Recently, congress passed a bill that extended the ITC at its current rate out to 2019. This is expected to help maintain the rapid growth of solar PV in the U.S. in the coming years. In 2015 it is estimated that over 7 GW of new solar PV was installed in the U.S., with over 30% of all new electricity installations in the U.S. through Q3 coming from the solar PV sector.
You can find a nice article that provides more numbers on recent solar PV installations, projections, and the ITC here.
The Solar Energy Industries Association (SEIA) and GTM research also recently released their 2015 Q3 report that is worth glancing at.