Well, as always, the devil's in the details.

A lot of info can be found here, although I haven't verified most of it.

The yield on the Davy Crockett warhead was about .01 kT, which is about two to four times the power of the Oklahoma City bomb. The typical suitcase nuke runs about 1 kT, although up to 10 kT has been mentioned. The Hiroshima bomb was 15-16 kT.

None of these could destroy an entire city, and if people were properly educated, people in the outer areas could survive the radiation. Fallout would probably be worse than Hiroshima, which was an air burst. A terrorist weapon would no doubt be detonated at or below ground level.

Most likely scenario is someday someone will set one of these off, and a lot of damage will be done. And we'll kick the ass of whoever does it. As I've said before, lets see who runs out of bombs first.

Personally, I would be more worried about poisoning water supplies or biological warfare.