Jack started in Toronto in 2002 and has since been licensed to a number of US stations. Their basic concept is that they play what THEY want, not what YOU want. The format is somewhat loose where each station can add their own flavor to the recipe. There are recommended playlists and promo clips. On the original Toronto station you are absolutely guaranteed to hear at least one Bon Jovi song every morning.
This whole reverse psychology bullshit appeals to the dumb-asses out there who suck it all in. They fail to realize that the "play what we want" is in fact the creedo of pretty much every station out there. They just don't advertise it.
For the majority of last year they pushed the fact they had absolutely no DJ's - in fact Jack and most other stations don't have DJ's but "personalities" anyway. But any talk time and nonesense from these personalities was replaced by commercials, not more music. They promoted this concept constantly - it, along with their primary "play what we want" was their edge over the competition.
In the past month or so they have brought back morning and evening personalities and are now advertising their morning show and drive home show. The personalities are, of course, obnoxious twats. Without them you have semi-decent musical selection of mostly older cuts, but very annoying commercials and promos.
My girlfriend keeps the alarm clock set to Jack, so depsite my no radio stance, I do catch a bit in the morning as I'm waking up. Once I pick up some powered speakers that can turn themselves on/off with a line-level signal, I'll be putting a Squeezebox in place as our alarm. Then I'll play what I want.