We did it last year. I didn't have to swear to the queen, not even God!!!
The affirmation (the Oath is the religious one) has you swearing your loyalty to Australia and its people, no mention of auld lizzie.
I love it here and am not going back so it was a no brainer for me, I just saw it as a natural step up from 457, PR then citizenship. I feel like I'd just be pretending otherwise. I've also got high school aged kids and it makes a big difference to University fees and the like.
Get it done.
plus the Auzzie passports are cheaper.