1) Do you NEED solid, consistant media exposure...week after week, or are you satisfied with "occasional" exposure? Now, this question alone is important... but not enough. The main component of this question is the IMPORTANCE of PR.
2) Do you have the internal staff and expertise to commit the internal resources to your PR efforts?
If you have the internal staff, and they understand Guerrilla PR principles, then there may be no reason to hire an outside agency.
Paradoxically, the busier you get, the easier it is to parlay, or "set aside" consistant, important PR activities. Don't get caught in that trap!
3) Finally, Public Relations is a craft that requires PASSION. You may need PR, and you may even have the people to conduct your PR campaigns, however, that's not enough.
In order to be truly effective, it's important that your PR campaigns are conducted with PASSIONATE CONSISTENCY.
Here's a quick "checklist" you can use to "size up" any PR firm you are considering to hire: