If you enjoy hardcore action, this is definitely something you should check out. In my view, Black Lagoon is one of the best action series I've seen in a while. Fast-paced, exciting, and packed with gunfights. And there exists the toughest set of characters to patrol the South Pacific and walk the streets of the fictional city of Roanapur (think GTA's Liberty City where just about everyone breaks the law).
Do not expect a plot with depth. The series is episodic. As BStar mentioned, there is character development as we watch Rock the salaryman learn how to survive with his new friends, but other than that there really is no all encompassing story. Instead, we watch the main group engage in several different missions throughout the series (they are hired mercenaries most of the times), each mission quite the explosive action adventure.
I loved it. It was fun.