While I would not characterize them as common, friends of mine who have served in Afghanistan have told me that their bases have been attacked in larger scale attacks similar to this one from time to time though perhaps not as large. The Fox News arcticle mentions that Chechens are known to operate in the area. That might explain the success of this attack. I am told that ordinary Afghans are not particularly spectacular fighters but the Chechans are brutal and very competant.

SFC W