What To Say?!

What is the proper thing to say when someone gets engaged? Congratuations seems so wrong! Like, "Hey, congralations, you finally snagged one!" I heard once that "I`m so happy for you both" is appropriate and I have said that, but mainly to people I know well. Is there anything more appropriate to say to someone you know, but not well that would not make me feel like I am cheering the champions for "making it into the club?"