Martin, I've just edited my original to say "too many", rather than "most" as I don't actually know (or even think) it is most, but I certainly know it is far too many.
What ever you do don't choose Vocus. We're currently in dispute with them as it is without doubt the WORST media database I've ever had the misfortune to use. They also have the most appaling concept of customer service I've ever come across - my complaint was met not with a response to the actual complaint but "big PR companies use it" - as if that makes any difference to the problems we're having. We wouldn't go back to a complaining client and say "well everyone else likes us" - we'd solve the problem.