Well I ended up buying one of these today but I honestly have mixed feelings about the handset.
Obviously, the display is brilliant but suffers from the same graininess at lower brightness levels that other AMOLED screens I've come across. It's probably even more noticeable on the Galaxy Nexus as the auto brightness levels are set relatively low in comparison to the Galaxy SII. I'm not fussed about this though because no screen technology is perfect and as I said, it's apparent on the Galaxy SII as well.
The user interface is definitely very nice and a lot more uniform across the whole phone and the new Google applications look fantastic, especially Gmail and YouTube. For most part the UI is very slick but I still came across certain applications which were definitely not as smooth as the counter part applications on the Galaxy SII.
The browser is one such example when visiting the mobile version of Androidandme.com. Flicking my thumb from the bottom of the screen to the top on that Galaxy SII had the page scrolling down to the bottom in a fluid and insanely fast manner where as the Galaxy Nexus was no where near as smooth. The difference really is night and day, the best way to describe it is like scrolling a web page on your desktop PC with and without smooth scrolling enabled.
I'm not sure why this is the case, seeing as every application compiled with the ICS SDK as the Browser on this phone would have been; should be GPU accelerated.
The speaker is pretty shocking both in its placement and in its sound. I find my fingers covering the speaker when holding the handset so both of my thumbs are free to control the screen; just as you would hold it when gaming. I had the same issue on the Galaxy SII but because the speaker was offset, simply turning the phone 180 degrees so that the Samsung logo was under the other hand meant the speaker was not covered. I can barely hear the notifications at full sound settings in a fairly quiet room and I'm so concerned about the alarm not waking me tomorrow that I've kept my SII alarm on as well.
Some of the UI changes are actually a step backwards, such as the recent applications list. Before you had a grid of icons 4 x 2 in size, allowing you access to the latest 8 applications without having to scroll. The new method allows 4 to be seen, just so it can be a little bit fancy and show a window preview.
Pretty sure I read that Google had finally got around to allowing the device to sync contact profile pictures greater than 96px x 96px in size but bringing up a contact's details in the People application still shows a blotchy low resolution photo from Google+. This is another annoyance, it appears the latest Nexus S Gingerbread update and ICS has taken away Facebook's contact sync privileges as before, Google allowed Facebook contact sync outside of the Google contact API. This means most of my contact's don't have display pictures because Google+ is about as popular as the smelly ginger child at school.
Friendcaster is a way around but again, the contact pictures look awful despite checking the high resolution option.
The gaming performance has been surprising, Shadowgun appears to run as smooth as it did on my Galaxy SII but I've only had limited testing due to a bug. After interacting with the first dead enemy in the game, where you send a tissue sample off, the ability to run whilst aiming disappears; almost as if multi-touch completely falls over.
Google have done a great job with giving Android the polish it's been screaming out for since it was first released but again, between them and Samsung; they've missed a trick or two on some of the basics in my opinion. Most applications in Android are not full screen apart from games, which leaves the notification bar to be accessed most times; so why on earth haven't Google implemented the quick settings toggles that you see in TouchWiz/Sense?