I put "common" because I've met a few people named Jack in my life, and it's a decently popular name from what I can tell. That's in real life, though.
In fiction, Jack is quite common, with me having starred in Jack and the Giant Beanstalk and Jack the Giant-Killer in terms of fairy tales. I'm also nimble, quick and good at jumping over candle sticks.
Jack is also generally used to portray a down-to-earth, straight-shooter, working-class kinda guy (Madworld's protagonist, Third Watch, several other television shows, Mad Jack from Heavy Rain, that sort of thing).
Also, according to www.behindthename.com:
Derived from Jackin (earlier Jankin), a medieval diminutive of JOHN. It is often regarded as an independent name. During the Middle Ages it was very common, and it became a slang word meaning "man". It was frequently used in fairy tales and nursery rhymes, such as 'Jack and the Beanstalk', 'Little Jack Horner', and 'Jack Sprat'. American writers Jack London (1876-1916) and Jack Kerouac (1922-1969) were two famous bearers of this name.
So, yeah. Sort of common?