According to the techies, of which I am not one, the stock Samsung ICS kernels can potentially brick the phone if you wipe (factory reset). You may get away with it once, or a dozen times, or a hundred, or it might bite you the first time you try. It's due to the firmware bug on the memory chip which can go tits up on receipt of a specific "wipe" command. Some kernels issue that command (Samsung official ICS kernels and leaks) while others (Gingerbread and some custom ICS ROMs) are engineered not to.
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I have had good battery performance with ICS, for a day or two, but sooner or later it all starts to go a bit weird and the frustration levels increase as your trust in the phone to behave itself diminishes. I've had the Samsung keyboard consuming obscene amounts of CPU time, or the system itself, or other things going into overdrive. The phone becomes very hot as the CPU is stuck at 1.4Ghz. The trigger can be leaving it on charge a bit too long or removing it from the charger or just looking at it a bit funny. There seems to be a bit of hoop jumping required if you want to try to avoid these problems, but I was getting tired of it all. Under GB these issues do not exist. Under ICS, more than once, these random irritants kept rearing their heads. I've tried freezing all sorts of surplus apps/services with Titanium Backup, but when crucial stuff like the keyboard app misbehaves you're a bit stuck, and I tried switching between Samsung, Swipe and Swiftkey, all to no avail. The best solution I found was to reboot the phone with nothing attached to the USB/charging port, but quite honestly that's unacceptable. It should all just work properly, as it did with GB. I should be able to stick it on charge and remove it again, when fully charged, without having to think about it.
I did briefly try going fully UK with my phone by putting on a UK GB ROM, changing the CSC to XEU and then trying an update to ICS through Kies, but since Kies refused to acknowledge the existance of an update I decided to abandon that strategy and returned to German GB, wiped, upgraded to German ICS OTA and had a couple of successful days of harmony before the rot set in.
Maybe the UK version of ICS is much improved, but the German one is definitely quirky. I can easily afford to stick with GB until an ICS 4.0.4 ROM turns up form Samsung. It's not even though I feel as if I'm missing something by holding fire. The ICS update didn't seem to bring much with it, not least because it's been Samsunged by Touchwiz and other non vanilla "enhancements". It was actually pretty much a damp squib as far as I could see.