Like Chronata, I believe Crows have been given a bad rap. The NW coast Natives have many stories about Raven and Crow, mostly raven (I don't think they made a distinction, as it is rather difficult to tell them apart).
In NW Coast mythology, raven/crow was the Trickster, the bringer of light to man, amd many, many other tales. In all, he shows his dual nature as an entity who tries to trick but ends up doing good.
In the "Oracle of the Dreamtime", a similar story is told about Crow. I haven't had an opportunity to independently validate it as Aboriginal, but believe it is correct.
I like crows. They are very family oriented. They pair bond; their young live with them for five years, they live up to 15 years.
I think seeing them says "Look out! You may end up thinking one thing and doing another", but maybe that's just my experience.
I don;t think a single crow is an omen - or I get plenty of omens!