The quick answer is that it's too soon to say. Obviously, the Clintons are one enormous package of needy egos laced with drama. As long as they are on the national stage, they are trouble for any other Democrat seeking or holding a national role, because they are people who can't stand to be upstaged.
Obama has made the calculation that they will do him less damage inside the tent than outside it. Inside, at least they both have a stake in the success of his administration. Outside, they'd have no reason not to cause difficulties in hopes of burnishing the relative luster of the Clinton Administration or furthering Hillary's national ambitions.
Will Hillary try to take over? Probably. Will she "do her own thing"? Of course. And will her husband create embarrassment for her and everyone else? Almost undoubtedly. What's more, any off-the-cuff utterance the utterly undisciplined former President makes about world events will be scrutinized for the extent to which it reflects Hillary's (or the Obama Administration's) policy.
But apparently President-elect Obama considers all that a fair trade-off for being relieved of the necessity to watch his back simultaneously at home -- with Hillary Clinton in the Senate -- and abroad, with Bill Clinton roaming the globe.
How's that old slogan go? You give me domestic policy, I'll give you the world. That seems to be the arrangement Barack Obama has tried to make with Hillary Clinton. We'll see if it works.