Note: this review covers my thoughts on both seasons of Black Lagoon and the OVA Roberta's Blood Trail.
The best way to describe Black Lagoon is dark and twisted fun. The show can easily swing between an over the top, bloody, violent, and action filled romp and extremely dark, psychological, mature, and a bit horrifying in the best of ways. The two lead characters are both very well developed and have one of the more interesting relationships I have seen in anime in a while. However, there are some side characters that I feel could have used some more development. Shows like this that go back and forth between two very different genres can often feel like they have consistency issues, but Black Lagoon pulls this blend of genres off better than most. The animation is great and the action is bloody and fun. The music is also pretty kick ass and the show has a really cool/interesting setting.
Black Lagoon is an easy recommendation for anybody looking for a darker more mature action series.