"When Ballmer became CEO in January 2000, Microsoft's stock was around $53. When he stepped down in February 2014, it was around $38."
By that metric, no he wasn't underrated.
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He took earnings per share from $0.70 to $2.63 during his tenure. A CAGR of 10.5% across 13 years in one of the world's biggest companies. He couldn't control multiple compression following the dotcom bust.