Pretty sure a blind test would result in no noticeable difference - it did for me.
I always used to put V-Power in, then drive around thinking I was in a Shell advert and my engine was working like a slick, well-oiled machine...bursting with more power. Definitely felt like it had more power.
What completely changed me was when I asked the other half to put Tesco Momentum in. There was I thinking my car was running super-efficiently etc. She told me a week later that she had put normal fuel in and my world was turned upside down.
Felt like I had a been punched in the heart. I went through the 5 stages of grief - Denial (how can premium fuel not make a difference?!), Anger (****** Shell!!!), Bargaining (maybe I still had 99 octane in the tank still so it didn't feel different?), Depression (all that money wasted...) and Acceptance (I'm happy with normal fuel).
Also, I have a 1.4 - it probably couldn't use premium fuel anyways (as V8 said above - depends on the ECU). Perhaps if I had a GTR, big AMG or an R8 then I'd probably put premium in as I think it would make a difference.