I upgraded from an HTC Evo 4G (the original Evo) and after a few days with the SGS3 I've run into a few things that make me a little disappointed.
1. The home button is mechanical instead of haptic. This makes it more difficult to use the phone one handed as the phone is very big and getting my thumb all the way down there to press on the mechanical button is awkward.
2. Only three buttons. The Evo has a fourth button dedicated to google search. This makes it incredibly easy to find whatever I'm looking for. A short press on the search button and the Google Search app is ready for input. A long press and Voice search opens. On the SGS3 I can double click on the mechanical button (somethign I've already noted as a weakness of the phone), and while that shouldn't really be a bad thing...
3. The S Voice feature in touch wiz is absolutely horrific. When the app loads it asks me what I want to do and I should be able to issue a number of different presets ("Search" or "call" or "message", etc, etc) but the software has really poor voice recognition. And any background noise renders it completely useless, as it responds to everything with "Say 'Hello Galaxy' to wake me up!"... Ahh... what? Why is that even the default instead of "I can't understand you."? I believe this issue stems from the fact that S Voice is doing all the processing locally on the phone. Which brings up the question, why don't they just tap into Google's own "voice recognition in the cloud" offloading. If you use Google Search's own voice recognition, on the SGS3 it works perfectly. You would think I would just program my SGS3 so that Googe Voice Search came up with a home button double click, however...
4. TouchWiz has practically NO customization options what-so-ever. I can change the wallpaper and I can move buttons around and that is about it. I can't even get rid of the TouchWiz home screen indicator that tells you what home screen you're on and consumes a whopping 1/5th of the phone's screen. Yes, its a big screen but come on. Why waste so much space on such a tiny platform with something so trivial?
5. The bloatware can't be uninstalled. I've encountered this before on other phones (including my venerable Evo) however the bloat on the SGS3 isn't just (annoyingly) present on the phone. Its obtuse and intrussive. Oh, you're trying to make a phone call? Let me push a pop up over the dialer app so I can tell you how you should buy Dropbox to save all your mobile stuff that you already save with Google Drive, even though you've already checked the box telling me never to tell you this again... 5 times... in the last hour...and you're stuck with me because the app is embedded in the OS and can't be uninstalled.
And these are just the biggest issues I have with the phone. There are many other gripes I have already encountered with the design and the software. With the exception of the mechanical button, most of these issues are surmountable, simply by rooting the phone and installing Cyanogen Mod however, the camera doesn't work in CM10 or 10.1. Its apparently a known issue that affects some, but not all, SGS3s. Lucky me. So I'm back to using TouchWiz just so I can get the camera working. And I hate it. I really hate it.
So, with that all said, there are some good things. The phone is fast, like REALLY fast. The screen is stunningly beautiful. Full screen video with the Netflix app is excellent. The pair of headphones they give you with the phone are actually pretty nice. My cell reception is better with the SGS3. Battery life is great (2 full days of heavy use on a full charge, something unheard of on my Evo even on day one). And LTE is very fast. As far as the hardware is concerned, the device is sound. TouchWiz... not so much.