I think a watermark looks good, done tastefully. Not a bright red blob of text over the subject's face. That said, I've never watermarked any of my pictures. None of 'em would be good enough to steal. xD
Generally speaking, I like watermarks because they let me find more pictures done by that person.
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COSPLAYS IN PROGRESS
Soren (Fire Emblem Radiant Dawn) - 9%
Vaati (LoZ Minish Cap) - 2%