I'm finishing up the fourth chapter of James in my sermon this Sunday. I've really enjoyed preaching through James, and have seen many important issues work through the life of my church.
The main commentary I've been using is the Pillar New Testament Commentary on James by Douglas Moo (who is obviously the long, lost twin brother of Stephen King with different writing goals).
I've also been using other commentaries. Kent Hughes, Thomas Manton, Peter Davids, and the New Bible Commentary among others. But none of them are as good as Moo. I'm very happy with it and encourage you to pick it up if you need a fairly critical commentary on James.