Just spent three days in Lisbon and a week in wider Portugal with only the Pancake lens (and a camera, hurr) and ... it was perfect.
Been wanting something smaller and lighter for a while, not had the money to pick up another camera by Fuji or similar - this transformed my SLR into everything I really wanted. Smaller and lighter than with a zoom, plenty discreet enough for street work, and I don't feel like I missed many shots by not having WIDE ANGELS covered - but that's not my thing. Focal length on a full frame body is great, between 35mm and 50mm - not too wide, not too tight. AF isn't astonishing but unlikely to let you down too badly on vacation, and it'll really let you enjoy it - based on my own personal experience.
It's cheap and very cheerful, no need to invest in a new system if you're not looking to do that.