My company prohibits dispatching to a "poor" braking action runway. However, once airborne, it is Capt's decision. General manufacturer guidline is x-wind to not exceed 1/2 of max (so 15kts) if the BA is poor. Also, the company SP's say that any BA reports we use as controlling must be from an aircarrier or large biz jet, or the airport authority having done friction tests.
Also, I make it a point that my ac type (EMB-120) is included with any braking action reports given. Almost always the ac type is included in any BA report given out.