I've used several virtual assistants from oDesk in the past 3 months for time sensitive tasks and couldn't have been happier.

With oDesk the quality of VA is a lot more variable so I would make sure to check out their ratings and past reviews.

Also a quick hack I used is hire 10 VA's for 1 hours each to do a small project or piece of a larger project. Review their work and pick the one who did it the best with the least self direction.