Hey Nexus, I posted some very short thoughts on the Nexus 4 in comparison to the S3 over in the S3 thread, here's a copy and paste job!
On another note I've bought the N4 late last week and have been using it, although it's a very good phone something's missing from it to the point that the user experience almost feels "soulless"
I can't put my finger on it but I feel that the S3 brings an all round user experience, little things like Smart Stay and other implementations that Samsung have placed in Touchwiz are just the icing on the cake.
The N4 is better built and looks classy but ergonomically the S3 is better to hold.
Sure the N4 will get all the updates first but Samsung hasn't been slouching on the update front and the latest 4.1.2 premium suite update brings a host of features to the S3 that give an enhanced experience.
Don't get me wrong the N4 is a great phone but I feel that the S3 is just a little bit better.
The battery life is the Achilles heel for the N4, it's positively poor compared to the S3, I can easily get around 19-20 hours with moderate to heavy use from the S3 and around 12-14 hours same sort of usage with the N4.
I don't know what it could be but maybe it's poor optimisation on the software that the N4 is running.
Screen wise I've always prefered the SAMOLED look compared to LCD, and that still stands, however the LCD screen is still very good and extremely detailed. It's just a matter of preference but I do prefer the S3 screen because the colours "pop" to life, while on the N4 they're understated but more accurate.