We saw them at Bumbershoot in '02, as well as Jon Langford's other band, the Waco Brothers in '03 (also saw Sally Timms solo as well). They weren't bad, but not worth a second time around. The Mekons were better than the Waco Brothers.
I remember the Mekons used to always play that the little hole in the wall alternative club when i was in college in the late 80s. Never saw them then, though I did quite like their "Mekons Rock and Roll" album.