I'm not sure if it's true (although it sounds like it could be), but I once read that there are only two national anthems in the world which extol the Heads of State rather than the nation and/or its people - ours, and North Korea's.
I'm a republican, I don't believe in god, I'm not into patriotism or nationalism and I don't believe in worshiping a Head of State - so I don't sing it. I wouldn't even call it a choice; I simply can't bring myself to sing those words.
But I've absolutely no problem with those that do want to sing it. I actually think it's a decent anthem because it's short (the common version), sweet and builds up. Can't stand those miserable Eastern European ones or those long, boring South American ones. Love the French and Italian ones, I'd probably sing those if I could.