I think clip-in ones are relatively widely used - I have 3 friends who have them for nights out. One of them is quite glam but the others are just normal (if that makes sense...)
I only know one person who has 'proper' extensions all the time and she works as a model so needs to maintain her appearance. Her extensions are amazing, they look 100% natural and I would never have guessed they were extensions if she hadn't told me. Who knows how many other people out there have them but you can't tell...?
Well, I have them all the time, and think I'm quite normal! Certainly not a model! I have alopecia, so have a 'system' to which the extensions are attached. When I tell people they are amazed, as they are really natural looking. I can use the Babyliss Big Hair, heated rollers and tonight I've dyed my hair. They are high maintenance, but after years of hair loss I don't mind spending the time or the money, as it makes me feel normal again, and they give me confidence I didn't have before.
Three of the hairdressers at the place I use have them and they look really good. I had to ask who had them because I wouldn't have been able to tell. I think the price, time and maintenance will put lots of people off though.