I came from the S3. I do not like Samsung, but the S3 was okay. It just had a bad radio. Samsung only let US Cellular upgrade so many, and US Cellular basically decided which accounts they could afford to lose, and replaced those accounts' S3s. Mine was one they decided they could lose. And they did. Now I have an M8.
First thing you're gonna notice, it's a taller phone. The screen is not noticeably bigger, but the body is taller. The speakers, front camera, and the HTC vanity bar all add to the height. No Home button in front (this isn't a second-rate iPhone clone, LOL) and the Power button is on the TOP. Yeah, have fun with that if you don't have large hands.
Second, it's slippery. Metal comes with a price, and your phone sliding out of your hand is your price. Once you've wooed all your friends with your shiny new metal phone, put a case on it. Or at least a wrap. You WILL need a case if you want to use a FlyGrip with it. Not many do but I do, so I thought I'd point that out. I like the kickstand aspect. My M8 is standing next to my keyboard. Looks amazing on my desk. Also holding it with the FlyGrip (which should be positioned below the HTC logo, and above the bottom strip—no more Verizon 4G LTE logo!) the top button is easier to hit.
No more removable battery. You can break the connection to the battery by holding all three buttons (Volume up, volume down, and power) for 10-15 seconds. I've had to do this a few times.
The M8 can be rooted, but there are extra steps. HTC uses a feature called S-On to protect the bootloader (HBOOT). So you need to temp root (via an app called weaksauce), gain S-Off, flash a recovery, flash SuperSU, then remove weaksauce. Don't follow my instructions; instead look for the thread on it. And on top of all that HTC locks the system directory. The stock/rooted ROM takes care of that for you, but it can be an annoyance. Like you can't use AdAway because it wants to write to system, and AdAway won't mount system as r/w. One solution I have found is, install Xposed, and that mounts system as r/w. When Xposed wants to reboot, switch to AdAway and let it do its thing. It will succeed. THEN reboot. But only the stock ROM you root yourself has this problem. There's a stock rooted ROM that has this dealt with. And then there's CleanROM which I haven't tried yet, but it's basically, as I understand it, the stock/rooted ROM but with a lot of the crap taken out.
TouchWiz is... okay, "If you can't say something nice don't say anything at all." So I will just say it is not a nice OS. Sense is very elegant, but limited. BlinkFeed is cool if you like the news feed thing. I personally don't. I'd rather just go to the sites I read in my browser. I don't need a feed built into my launcher. But at this point, I actually prefer Sense to the AOSP-alike ROMs. Sense works. The AOSP-alike ROMs, at this point... don't. They have various issues that you may or may not want to live with. But I'm not knocking any developer—I am sure they are all working hard to give you the best experience. Just keep in mind that this is a 3-month old device. Right now everything is wild, wild west in terms of development. The best ROMs are Sense-based because that's what's stable. That said, we do have a GPE (Google Play Edition) port and that's what I'm running now. I opted for stock everything in the Aroma installer and it's not 100% stock (the original is made by a team called SkyDragon, and they put some of their branding in it) but for the most part it's turned my M8 into a Nexus phone. And stock Android is really cool. But it's not without its faults. For example it handed over the audio from Poweramp to a call, but when the call was done, it crashed and I needed to three-finger-salute it back to life. That's not uncommon.
I think the Galaxy S3 was more stable on the software side, but then again I did not own one until ~11 months after release. And then waited a month to get into custom ROMs. When this phone's development community gets strong, I think we'll have it real, real good.
Oh, and about the speakers, yes they are as awesome as you think, but they won't blow your mind like a car stereo or boom box. Maybe a $30 Walmart CD player. But for a phone, that's impressive. Just my opinion, but polyphonic ringtones still sound the best. Search Zedge for "8 bit" and see what comes up. I have the Game of Thrones intro rendered in MIDI for my ringtone. There are two I've heard. I'm using the good one. For my wife, I'm using a MIDI render of the Shingeki no Kyojin (Attack on Titan) theme. It's really good. Sounds amazing with these speakers. But I'm old school, I think a ringtone should be a ringtone and not a part of a song. I don't think any small speakers have the range to appreciate music. But maybe I'm just a snob. I want to watch a movie on mine, and see what that is like.
Anyway, the S3 is a 2 year old phone now. If you get good signal on yours, you don't really need to trade up. It runs KitKat just fine. It's gonna get L, at least unofficially if not officially. But if you want the new shiny, in my opinion the M8 is the best Android smartphone on the market right now. The 3GB G3 might beat it, but I think that QHD screen will work against it in terms of battery life. The M8, by the way, has excellent battery life.
