The chief advantage of incumbency is that of name recognition. An incumbent running for re-election no longer has to go through the long, arduous, expensive business of trying to establish their name in the public eye. During their time in office, incumbents have constant access to the media, giving them the opportunity to get their message out and to justify their positions on the political issues of the day. As elected officials they're also entitled to a substantial grant of tax dollars to help them communicate with their constituents.
It's often been said that money is the mother's milk of American politics. Being an incumbent gives you easier access to the funds necessary to carry out a re-election campaign. Potential donors no longer feel that they are taking a chance on a rookie candidate; now they're backing someone who's already been tried and tested in the bear pit of political life.
As to the disadvantages, well name recognition is a double-edged sword. Lots of people may know all about you, but familiarity breeds contempt, as they say. Once an elected official has gained a particularly negative public image, then it's nigh impossible for them to change perceptions. If there's a desire among the electorate for something new, then an incumbent can look decidedly old hat.
A negative track record is also a disadvantage. Your opponents can go through your policy statements and votes with a fine tooth-comb, highlighting anything they think can be used to undermine you.
Voter fatigue is another potential pitfall for an incumbent. If the economy's in trouble, or if there's a general sense that the country's going in the wrong direction, then incumbents are particularly vulnerable to a public backlash. They're part of the political establishment, and as such the voters might very well be keen to give them a bit of a kicking come election day. The word "new" is one of the oldest in politics, and when there's a sea change in the air, it's pretty hard for incumbents to do anything about it, despite their manifest advantages.