Since I sell nationally I might be able to help.
JACK's launch well when supported by huge amounts of television
advertising. Such was the case in Dallas, Denver, LA, St Louis
Portland and Seattle. In each of these markets JACK peaked at top
three in 25-54 adults. St Louis had, by far, the biggest launch of
all.
At one year the JACK stations begin a rapid decline. Denver and Dallas
were the first two JACK's and are 50% of their peak numbers.
The JACK's that don't launch well, generally failed to use enough television
advertising to support the station, or the market was covered by a number of
rock leaning stations that already provided enough variety. JACK has also not
done well in markets with huge urban stations. (Atlanta, Washington DC, Memphis
etc)
JACK seems to be following the pattern of the 80's stations five years ago. Seattle, Portland and St Louis saw the biggest launch of these stations as well.
Hope that helps. Advertising Age, not too long ago detailed the dangers to
media buyers purchasing JACK stations. They are the latest fad, not much more.