Who has the Advantage?

Here's a position from one of my recent over-the-board games. The next move developed a decisive advantage for black, who went on to win. But was this result inevitable? Black's king seems more vulnerable but he has well-positioned pieces. White's a pawn up, but is he well defended or cramped?

I'm unclear whether either player has a definitive advantage. What do you think? What's the best move for black? What's the likely result?

Was the black victory that transpired the result of poor defence by white? Is there a killer sequence that will defeat black?