Im in the process of leaving Vodafone, they have offered my a Free S4 or Experia Z for 29pm on one of there Red Tarriffs, Unlimited Mobile and LL Talk, Unlimited Text and 2gb Internet, 24mth contract which I was considering.
Having spoke to a couple of people with these phones now, I decided not to bother, the S4 is only 16gb memory, so by the time Samsung wack all there extra silly features on top of Android, your only left with a few gb's left for your Apps, Music and Videos. the 32gb Version is coming out soon. The Xperia Z I quiet liked for its Screen Size and expandable memory, but just couldn't live with TFT screen brightness, you have to really look at the screen head one which got worse under bright light.
Personally I think its the wrong time at the moment, to sign up on a 24mth and a 4g mobile contract, give it another 6 months and Vodafone, O2 will be launching there own 4G services, together with 3 (on the EE Network) and then the competition will start to crank up a bit and we should see some better deals to be had, not to mention more 4G mobile choice on the market.
When I declined the Vodafone Offer, they offered me a 12mth Sim Only deal, 600 Any mins, 750 Voda mins, Unlimited Text, 500mb Mobile Web and 1g WiFi for £10.50 a month, so being only a 12 month contract I dont mind dumping that halfway through or do an Upgrade in 6 months time.