I'm reading and following along with the study over at www.challies.com. We're on week 2, and Tim has a good summary of chapters 2 and 3 there. While all of the study is very, very good and helpful, this quote in particular struck me:
"The purest actions of the purest men need Christ to perfume them; and this is his office."
Those following along with the study are working on a definition for the bruised reed and the smoking flax right now in the comments section. I take it that the bruised reed is one who is acutely and chronically aware of their own sin nature. Not just particular sins or behaviors that are sinful, but our indwelling bent toward self and away from God. When the sinfulness of our sin is pressing down on us, it bruises us even more deeply. And that fullness of our bruising is what was laid on Christ as He suffered on our behalf on the cross.
As for the smoking flax, I find that just a bit more allusive. The smoking flax is like the lampstand that has been lit, but is smoldering and hasn't quite caught fire. It emits smoke instead of light, which smells, chokes us, and can be downright offensive. Christ will not put it out. Rather, the Holy Spirit will come along and fan it into flame so that someone who is a smoking flax will soon emit light and heat, as well as a sweet fragance -- the aroma of Christ.