Yesterday bought Samsung S4. A few notes:
1. I found it on Gumtree for 380 GBP. Best way to buy in my opinion - people get it for free with an upgrade and sell unused (guy has Iphone himself).
Did not haggle but told the guy that he would have to come and meet me in Central London - he agreed.
2. Currently trying to sell my S3 for 250-270 on Ebay. If it sells, the upgrade will cost roughly 150 GBP - I think it's worth it.
3. S4 is noticeable faster & screen is just beautiful. My old S3 was getting pretty laggy - but then again I push it hard (like talking on Skype while editing photo in Snapseed). At the end of the day these lags are more annoying than anything - but since you spend all day with the phone, may as well have one that doesn't annoy.
4. I immediately put Google unbranded ROM 4.3 - one that comes with S4 Google Play edition. Did it assuming that it would make a better phone - unfortunately was not the case:
a) It was missing a few UX touches that I got used to - ability to switch Bluetooth/WiFi/Mifi Spot straight from notification bar
b) Ability to reboot from the power button menu
c) Lock screen that I've got grown to like
Overall the look on Google unbranded OS feels unpolished now, compared with Samsung one.
The advantages didn't seem at all important - I can easily hide all the Samsung crapware and they don't seem to impact performance. And I can't tell any difference between Android 4.2.2 and 4.3.
So after trying the Google ROM, I went back to Samsung stock one.
5. Ordered ZeroLemon extra-size battery (20 GBP) - 7,500Mah - or exactly 3x the size of stock battery. Should last me over 2 days if need be.