It depends really, it can be mysterious if the character isn't described for a while, or at all, and the reader can create what they think the man/woman looks like themselves.

However, sometimes it's nice to know what someone looks like too.

There's nothing worse than when a story starts out, "She had blonde hair, a tiny waist but with huge DD's" and that's all the description you get of a characters; it's best to just try and integrate it within the narrative ... like saying "Blue eyes glazing over in fear, she moved a blonde curl off her cheek, smiling nervously".
Maybe not as packed with detail suddenly, but along that line