Since then I've read a fair amount of Bonhoeffer, and have found it by turns hugely challenging (Discipleship), helpful (Life Together), wrong (Sanctorum Communio), and comforting (Prayerbook of the Bible). But I mostly love his Ethics. Writing at arguably the most difficult time for the church since Nero, Bonhoeffer grappled with the questions of what the church should do and say. Ethics in those circumstances is far from an academic pursuit! It ought not to be for us, either - like Bonhoeffer, we see the church in a storm.
For Bonhoeffer, ethics is about listening to God and doing what he commands. To be clear, this is absolutely not a divine command theory; I may have said this before, but I strongly suspect that whoever came up with the idea of a divine command theory had never encountered a divine command. This is not a system of principles. It is hearing and obeying. That is why Bonhoeffer could campaign for the churches to push a pacifist line in the run up to war, and yet join the conspiracy to assassinate Hitler in the 1940s. Far from being inconsistent, he is consistently seeking to listen and to discern God's will. (Which, incidentally, is why he should indeed be remembered as a martyr).
Perhaps the biggest thing we need to learn from Bonhoeffer is that the church has to act, and has to take a stand. That was the failure of so much German Protestantism around Bonhoeffer - either a complete theological collapse in the face of Nazism (leading to a failure to take any sort of doctrinal stand), or a moral collapse (leading to a failure to act on the doctrine they believed). The question for us really is: where will we draw the line? And having drawn it, will we stand on it?