I have two flights booked to Greece this year, both with a 15 kg limit. As I usually travel light, I thought this wouldn't be a problem. However, this thread has got me worried, so I have just thrown some stuff (including a pile of books) into my case and weighed it - 12 kg. So I think I should be OK. For me, the extra charge for people with extra luggage is better as I am not paying for something I am not using.
Greecemad