This slang is heard all over Spain, and constantly in Madrid. It's informal, but not rude.

Slang	Meaning	Example
guay	cool, great	¡Qué guay este sitio!
molar	to be cool, to appeal	Me mola tu chaqueta.
chulo	nice, cool (about things)	¡Qué piso más chulo!
chungo	dodgy, bad, tricky	El examen estuvo chungo.
flipar	to be amazed, to be stunned	¡Estoy flipando!
mogollón (de)	loads (of)	Había mogollón de gente.
rácano	stingy	No seas rácano, invita tú.
caradura	cheeky so-and-so	¡Qué caradura! Se ha colado.

• Guay doesn't change for gender — un sitio guay, una fiesta guay — and its plural is guais. You'll often see guays in messages too, but guais is the standard spelling.
• Chulo about a thing is a compliment; about a person (es un chulo) it means cocky.
• Invitar — here, to pay for everyone; colarse — to jump the queue.