
It gets a lot of laughs.

The Mini I recently bought for my wife had "You still look good in a Mini" on the back window. (A take-off from a 1990's advert for Special K breakfast cereal where the bloke's wife puts on a Mini-skirt) Sarah demanded I take it off.
My Moke has the classic "Moking is not a Wealth Hazzard" on it, taken from the mid-70's Moke adverts.
There is a guy with a grey late model Ford Falcon around here with "No, it's not my Mum's!" on the back.