The settlers of the West Bank have accomplished a great many things: They have built entire towns on previously-barren hilltops; they have created a network of schools, religious institutions and cultural centers that rival their counterparts in Israel proper. Of course, they've been subsidized generously by successive Israeli governments, but still, the concrete achievements are large. And in the political realm, they have achieved disproportionate influence, through savvy lobbying, clever coalition-building, and appeals to Jewish pride and tradition.
Their greatest achievement, though, is in the interconnected realms of ideology and propaganda. The settlement movement, its supporters, and its apologists (in Israel and in America) have successfully conflated support for their movement with support for Israel and for Zionism itself. They have created a reality in which criticism of the settlement movement has come to equal criticism of Israel. You see this at the AIPAC convention, where no speaker dared suggest that the settlements are, in fact, the vanguard of Israel's dissolution, rather than the vanguard of Zionism. (I explain why the settlements could lead to the end of Israel here.)
It is astonishing that what was once so small a movement now defines what it means to be a supporter of Israel. The official position of this blog (yes, we have official positions here) is that the settlements should be fought as if there was no such thing as anti-Zionism, and anti-Zionism should be fought as if there were no such thing as the settlements. This, I think, reflects the centrist position. A centrist on the question of Israel believes that the settlements represent a corruption of Jewish ideals, but that Israel remains the physical manifestation of a righteous cause. The right, of course, believes that settlements are an expression, not a corruption, of that cause. The left, on the other hand, believes that settlements are a manifestation of Zionism's true nature. I disagree with that argument strenuously. But I will say this, though: The left position on this question has the wind at its back.