He’s accomplished more in two years than the last two presidents did in 16 (whether you like what he’s accomplished or not). A huge chunk of the democratic agenda of the second half of the last century, largely ended the war in Iraq, got us actually INTO the fight in Afghanistan where we should’ve been in the first place, saved the economy from a second depression (though I sure wish it would recover faster already), and put a lot of money/effort towards shifting us to alternative fuels and upgrading our infrastructure (that stimulus will be getting spent for the next 10–20 years).
His popularity is dipped at the moment (in the 40% approval range) but you have to realize thats just approval ratings. There is a certain amount of people in the country who will vote for him regardless of how they think he is doing, simply because they will find the future Republican candidate worse. Plus most every president in recent history saw their popularity dip far lower during their first term and still found themselves re-elected (Bush 2, Reagen, and I THINK Clinton).
A lot of liberals/dems aren’t happy with how much he gave up trying to compromise with the Republicans, or how he’s avoided certain issues to this point (gay rights, immigration largely, etc)... But when it comes down to it they’ll take him over a John McCain or a Huckabee or a Gingrich in a heartbeat.