Right now he's a disadvantage for the party. He didn't keep most of his promising that got him elected. Get out of Iraqi and Afghanistan, he tried to stay in Iraqi and we're still in Afghanistan, to close gitmo. Then he pushed through the affordable care act, with the help of Nancy Pelosi, and her famous statement “But we have to pass the bill so that you can find out what is in it – away from the fog of the controversy.” the basically failed launch of the web site. Democratic's know this, you're not seeing Obama standing next to too many politicians, the left core of the party is mad, the republicans, have lots of talking points if they try to stand with Obama, so they distance themselves from him.
Despite the controversy around him, I would say that Barack Obama has been a clear advantage.
First: Obama has allowed the Democrats to solidly claim that they are the party of diversity. To their credit, Republicans have now run several ethnically diverse candidates and several women. (They brag far too much about this given the clown car that is running for 2016: It's easy to be diverse when you have enough candidates you need at least two hands to count them). This is a huge advantage as the country becomes more Hispanic.
Second: Obama was substantially more progressive than Clinton. While Obama has been very limited in many ways, he did win many major changes like the ACA (whatever its flaws).
Third: Obama has been able to challenge the stereotype of a stuffy liberal. The successful candidates for the Democrats have been cool in some way: Clinton was cool, Gore was not, Kerry was not, Obama was. (As an aside: This year, I really appreciate how Sanders has come off as the cool grandpa. He doesn't quite understand what a fist bump is, but you don't care).
Fourth: Obama's message of hope has galvanized a generation of more and more progressive youth voters. Millennials are largely rejecting the Republican Party.
Obama's election may be viewed as a watershed moment in the history of the Democratic Party within thirty years.
The older presidents were better. George Washington, Abraham Lincoln.... Those are the people who made America what it is today. Now we're dealing with people like Obama. We had much better people in the old days, and that's not just political.