Very much an opinion. I as well as many of my friends and family agree that 6tag is better than Instagram. There are many examples, this is imo the greatest example.
If it works, use it. If it does not work, don't use it.
Never mind about anything someone else has. You are falling into a media-hyped problem that should not matter if you enjoy the UX (User eXperience) on Windows Phone.
no idea, the apps I have work for me, I don't care what other people have. I know the operating system and cameras are better than anything I've used before.