im not an ex-galaxy s2 owner as i *still* own one alongside my iphone 4s.
things i dont like about it hardware build-wise:
- extremely surprised when i popped off the back of the phone to put my sim card in it. the backing is literally this wafer thin insanely cheap feeling plastic cover.
- all the buttons are super cheap plastic feeling
- screen has an obvious blue tint to it compared to my brothers htc sensation
- not convinced that bigger screen automatically means better. maybe i still have youthful eyes or something but i haven't found anything to be easier to read etc compared to my iphone 4s.
software-wise:
- well its still running android 2.3.x seeing as theres no ICS version for it either custom rom wise or official. i have to wait till april 2012 for an official one and who knows when a bug-free custom rom will be ready. currently i'm running the latest MIUI build and its B-U-G-G-Y.
- scrolling and zooming feel wholly unnatural. natural inertia on it is completely wrong. zooming definitely isn't smooth in the stock browser.
- apps, quality-wise are still ****. lots of big companies are coming along just fine with major apps so thats good, but all the main popular 'little' apps like ES File explorer STILL look like complete garbage and looks like it was made for windows 3.1. i don't know why there doesn't seem to be ANY design focus for android apps at all and is a huge turn off, especially after you skinned/customized the OS to your liking and then have to open up the ugliest apps ever.
i could go on and on but if you have any specific questions feel free to shoot.
This does not sound like a reasonable comparison at all. Thin plastic on SGSII back beats iPhone 4S' glass any time. Nobody complains about this plastic breaking (and even if it did it would be a $5 replacement). On the other hand, SGSII is really light. Feels great in the pocket. After SGSII, iPhone feels in a hand as a brick. Some call this feeling as caused by "premium build"

But it's just weight. One can easily attain this feeling on SGSII by slapping a couple of heavy nuts with Duct tape (not that I am advising this).
Same goes for "cheap" plastic buttons. There are much fewer complaints (if any) about those buttons from SGSII owners than from IP4 owners about stuck/worn out "premium" home button on IP4.
SAMOLED+ screen is universally recognized as the best mobile screen out there as far as colors are concerned. All other screens compared to SGSII look bleached.
Screen size affects not only your eyes but also your ability to control/select objects on the screen. WEB browsing on IP4 is comparatively hard exercise because of its minute screen size. It requires a lot of zooming in and out.
Whenever people start talking about "unnatural" inertia it means that they have just switched from one phone to another and new interface feels differently. There is no such thing as "natural" inertia for GUI. It's just what you are used to. SGSII GUI is extremely responsive. Scrolling in browser is faster than on IP4 (some call it "unnatural" inertia but those who switches from SGSII to IP4, not surprisingly, think opposite).
Judging quality of Android apps based on file managers is just stupid. Firstly, alternatives on iOS do not exist. More importantly, these apps are niche utility apps produced by small companies and offered for free. When it comes to applications that matter (Angry Birds, Google Maps, Google Navigation) situation is totally different.