A few days ago I read a article that said that the WTC attack was the equivalent of approximately .25 KT. Of course, that was spread out over, what, an hour or so? A nuke would deliver its damage over the course of a split second, rather than over the amount of time it took for the jet fuel to burn.
To put that in practical terms, I can tap you lightly on the cheek a hundred times in a row, and you'd barely feel it. But if I delivered the same amount of energy in one blow, you'd be sent reeling.
OK, so now that we know about how much energy was released on 9/11, and know what level of damage it caused -- and know that the damage would have been quite a bit more severe if the .25 KT were delivered at once rather than spread out over a protracted period of time, we've got at least a rough benchmark of sorts.
According to the FAS site mentioned in post # 109 in this thread, the ex-soviet suitcase nukes have a yield of 10 KT. So, a "suitcase" would have about 40 times the yield of the WTC attack, except, of course, it would be delivered instantly rather than meted out over an hour or so.
Not nearly a "city buster", but nothing to sneeze at either.
Since then, I've heard very different depictions of the capabilities of the so-called suitcase nuke.