This phone is awesome. As far as appearance goes, it's *thin.* Like, 8.49 mm thin. Much thinner than the iPhone 3G that I owned previously. Although it's thin, the curved sides allow for easy gripping. The casing itself is plastic, so it feels cheaper than other all-aluminum phones, but its absolute thinness prevents if from feeling cheap. It's a classy phone.
The screen is massive (4.3") and beautiful. It's probably the best single feature of the Galaxy S II. You can view the display from all kinds of angles (good for sharing pictures of cats) and the colors really pop. This is all due to Samsung's patented Super AMOLED Plus technology. It's the secret juice that makes the Galaxy S II's screen really stand out when compared to other phones. One semi-downside of the display is that it's resolution is 800x480 and NOT qHD. This means when watching HD movies you have to deal with annoying black bars around the film to maintain the viewing format.
The awesomeness of the screen, though, really can't be overstated. It's vibrant. Pictures and video are vivid and really pop off the screen, even in direct sunlight. The viewing angles and contrast are great. Really, a superb screen. Blows my old iPhone 3G's screen out of the water. It's a beauty to behold. Web pages render stunningly.
Samsung's S-AMOLED Plus technology also helps with battery life by selectively powering down black pixels to maintain battery life. This means that even though the phone has an impressive two cores clocked at 1.2GHz each and that beautiful 4.3" display, battery life is still impressive. I manage to go two days between charges and that's with fairly frequent usage. I do notice that taking a lot of pictures seems to drain the battery life, though, so you'll want to keep that in mind if you have cats. You should be able to get at least two days of battery life, I'd say. The Galaxy S II is certainly no slouch when it comes to the battery life department.