Absolutely voting matters. If people didn't show up to vote, the Iraq War would probably still be going on. There would be no Fair Pay Act for women. There would be no social security and public education, no health care reform, no end to top tax breaks, and the list goes on. The key is to vote in EVERY election, not just presidential elections. A president is not a king.
While a president has control over appointing Supreme Court justices, many things only are within the jurisdiction of Congress and on the State level. And because not many people voted in 2010 and 2014 and Republicans control both Houses of Congress, this President hasn't been able to accomplish as many pieces of his agenda as he wanted to (like a jobs bill that he supported in 2011). It's easy to be cynical and say that voting doesn't matter, or that Obama hasn't done anything for us, or that both parties are the same/controlled by the rich/whatever, but it's tougher to consider the policy-making process as well as the big picture when it comes to the differences among the parties and elected officials.