Hillary's probably the strongest runner-up we've ever seen in a national political campaign
and I was impressed how forcefully she leapt to Obama's defense following Bush's mindless comments about appeasing Nazis... She's definitely a team player.
She and Obama actually have few major policy differences - it's rather a stark contrast in style, personality, and appeal. They do complement each other very well, and an Obama-Clinton ticket would in all likelihood completely crush McCain and the GOP this fall.
The VP selection usually has a negligable impact on the Presidential race, but Hillary Clinton would be a big exception to that political rule.