Yes, we have Julie, a couple of times. It's best to get there early, as there will be hordes of people (and you have to queue up in a long line and be body-searched by nice police people before you're allowed near the church!! Actually, it's only a very quick pat-down, nothing dramatic, lol). It's a really good atmosphere, you quickly get chatting to the people around you, some of whom have come miles to see the Royals (the last time we were there we had some Americans beside us who had come to England especially to go to Sandringham on Christmas Day, and who were absolutely over the moon with excitement when the Royals appeared).
The Royals (the entire family.....it was lovely to see the younger ones, especially Prince William) don't hang about on their walk into church (the Queen doesn't walk, she goes by car), so if you want to see more of them you'd be better off going later for when they come out after the service, as they do stop and chat then. (Or just hang about until then, as you may not be able to get near enough if you arrive later!).
But as I said, it will be very busy! (but worth it).