I've always felt it best to use original imagery in marketing materials. Using a photo of an actual client or member of your team or place of business will always lend a greater credence and level of honesty than stock photo #2453 ever could.
This feeling was justified a couple weeks back when I attended a business trade show. As I was skimming through the materials that had been handed to me, I was hit with a sudden wave of deja vu. Sure enough, two different companies had advertisements that were being distributed to attendees at the show, and both ads used the same model from a series of stock photos.

Marketers rely on your suspension of disbelief. They need you to see the individuals in the ads as being real people like you or me, not a paid actor. If the same actor is talking about their love of Brand X and then is seen a few minutes later extolling the virtues of Brand Y, both brands lose out as the credibility of their spokesperson goes out the window. Likewise, the effectiveness of both these companies' ads was diminished by using the same model. The suspension of disbelief is shattered. We no longer see this as a fellow screaming in frustration at a tech support problem or smiling over a closed deal – this is just some photo from a cd rom. A bit of clip art. An act.
True – this was a fluke – and with the given number of stock photos, the odds of this happening in the same market during the same timeframe are low. But when you've spent thousands of dollars trying to differentiate yourself from the competition, trying to establish a unique brand, do you want to risk it all for the few dollars you can save by using stock photos?