As the dynamic duo steering Microsoft together for the past 28 years, Bill Gates and Steve Ballmer have been a near-unstoppable team, combining Gates's technical vision and will to power with Ballmer's salesmanship and rousing, if polarizing, personality.

Dorm-mates at college, best man at each other's wedding, their partnership changed in January 2000, when Ballmer took over as CEO from Gates, who became the company's chairman and chief software architect. And it will change again at the end of this month, when Gates retires from his day-to-day role at Microsoft, completing a transition process announced two years ago.

Gates will continue as Microsoft's chairman, and he told reporters at The Wall Street Journal 's All Things Digital conference last month that he plans to spend 20 per cent of his time working on Microsoft projects.

But once Gates leaves as a full-time employee, "I'm not going to need him for anything. That's the principle," Ballmer told the Journal earlier this month. "Use him, yes, need him, no."

Meanwhile, Ballmer said in a speech earlier this month that he plans to run Microsoft for another nine or 10 years. He will be 62 years old in 2018, and if he's still running Microsoft then, he will have been atop the company for 18 years -- an extremely long run compared to most Fortune 500 CEOs.

Some observers think Ballmer is up to the task.

"He's still got the energy -- I wish I still had that -- and the vision," said Tim Bajarin, ana analyst at Creative Strategies.

"Ballmer is a competition addict," noted journalist Fredric Alan Maxwell, author of the 2002 unauthorized biography Bad Boy Ballmer . "I see him giving up the helm akin to Charlton Heston giving up his gun -- 'from my cold, dead hands.'"

But other Microsoft watchers have increasing doubts about whether Ballmer, as a solo act, is the right person to steer the software vendor through the many competitive perils it faces in the Web era.

"He's still doing a sales job and not focusing enough on the rest of the business," said Enderle Group analyst Rob Enderle. Ballmer has neglected Microsoft's operations and failed to make tough decisions, such as firing underperforming executives, Enderle said.

George Colony, CEO of Forrester Research, sees a subtle slippage in Microsoft's standing vs. rivals such as Google and Apple as Gates has disengaged himself from the company -- and implies that the slippage could accelerate once Gates is even more out of the picture.

"Why hasn't Microsoft caught Google? Why has Steve Jobs clawed his way out of his grave to be adored once again?" Colony wrote in a June 16 blog post. "It's because Gates over the last five years has moved on to philanthropy -- and taken his formidable legacy with him."