I've used a foam roller for years, and though it works great for certain muscle groups, it's just not shaped right/too big to get into certain areas. I've had more than one guy on here recommend using a lacrosse ball for my pecs/shoulders, so I finally bought one last week and tried it out. Gotta say, I feel like an idiot for not having one years ago. Works so much better in those hard to reach trigger points. You can grit down much harder since the ball is pretty dense, but there's just enough elasticity that it doesn't feel like you're using a rock.
I was able to find knots I didn't even know I had.
If you do lots of self massaging, I really recommend getting one.