"Is she... isn't she?" was the catchline from the 70's advert for Harmony Hairspray which featured a girl with big hair walking down a road and two macho type guys saying, "Is she or isn't she?".
Apparently they were trying to guess whether she was wearing hairspray! Two guys, one pretty girl and some hairspray, hmmmm. I was 14/15 at the time, and although I never used this catchphrase as a pick up line I know some of my friends did...how sad! Having said that, they swore blind it worked... yeh right lads. But then, it was the 70's!