Photo Booth is a wacky and fun app and it’s tastefully done. It’s also an app people like to play with, it’s not something that’s used to get work done. I see no problem with it.
I absolutely do not believe that every app has to look the same. It’s ok for them to look different. It just (1) should be well done, (2) standard UI elements should be immediately recognizable and the look shouldn’t (3) create wrong expectations or (4) limit the UI in some way.
I personally don’t like the look of iCal and Address Book, I don’t know how others feel. (1) is certainly a very subjective point that can be endlessly argued about. It’s all about fashion.
iCal, Address Book and Photo Booth do very well on (2), but I think that iCal and Address Book do very poorly on (3) and (4).