[One (M8) General] jtc276:
Here's a personal mini-review and comparison to the Galaxy S5. I own neither (my M8 will arrive on Friday), but I played with both in an AT&T store for an amount of time that appeared to make the employees worried. Anyway:

M8:

Gorgeous phone, gorgeous UI, gorgeous everything. I'm a stock Android guy. I used to firmly believe that if Google were ever to release a Nexus device with a decent amount of storage (64gb at least) I'd jump ship immediately. Sense 6.0 has me rethinking that entirely. Talk about fast, intuitive, and beautiful all around. I used Sense 5.0 with the M7 last year before converting it to a Google Edition and thought it was "pretty good." Now it's by far the best Android skin and possibly better than stock Android in terms of features, consistency, and speed. The added gestures to turn the screen on, go to the home screen, open BlinkFeed, etc, all work wonderfully and truly make the device easier to use. And the camera app is an exemplum in app designs. Each feature is no longer hidden or hard to get to like in basically every UI's camera app. It's all very intuitive and simply makes sense (pun intended).

As for the actual performance of the camera...it seems to be alright. It definitely isn't better than the S5's, but it's not horribly worse. It's at the point where they're at least worth comparing. But, with the added simplicity of the camera app, it's much more enjoyable to take and edit photos on the M8 in comparison to the S5. But, yes, the S5 does produce better photos.

The design of the phone is, of course, great. I won't go into massive details over that because I realize that everyone here is already aware of that. I will say that getting used to the seemingly pointless HTC bar isn't too hard. And I'll also add that the M8 is hardly larger than the S5; it's longer by probably half a centimeter. And you have to consider what you get with that extra half a centimeter of length: two amazing BoomSound speakers that are better than any others on competing smartphones.

S5:

How do I say this without sounding horribly biased? Ok, I can't. The S5 is ugly. It just...is. I didn't actually expect my AT&T store to have it today and I wasn't even looking for it. While scanning the plethora of Samsung phones it just happened to be one of them. My first reaction to the device was "Oh, this must be another S4 variant or something." And then I read the name and realized that it was actually the S5. Being the white variant, the side bezels were all the more obvious and struck me as being very unattractive in comparison to the S4 that was sitting beside it. Actually the entire device was very unattractive when sitting next to the S4 and, let's be honest, the S4 isn't the Kate Upton of phones. The dimpled back, while feeling alright, was atrocious to look at.

But, enough with looks, how did the S5 actually perform in operation? Really, not too badly. TouchWiz, while still bloated and aesthetically vomit inducing, wasn't that laggy. A few areas did produce hiccups such as opening a folder or quickly exiting an app but, for the most part, it was much better than the S4 in performance. That being said, the way the UI is presented isn't as intelligent as Sense 6.0 or stock Android. Icons, while flat, just don't blend in with the design language of Android like Sense 6.0 does. And the camera app is still a mess, although the pictures produced are a bit better than the M8's.

The screen of the S5 is basically the same as the S4's in appearance. I know that it's apparently 20% brighter and more accurate, but I really didn't see it. I'm not the biggest fan of AMOLED displays and the S5 hasn't made me a believer in the technology yet. The whites were rather yellow and (I don't know exactly how to explain this), the depth of color in darker images just wasn't there. The M8, while not considerably better than the M7's, it still better than the S5's. Whites are beautiful, colors are accurate, and the screen is blindingly bright. Both the displays of the S5 and the M8 were at 100% brightness, so the comparison between the two is fair.

And last but not least: the speaker. Although I didn't compare (and I should have), the S5's speaker seems to be worse than the S4's. This is more than likely caused by the waterproofing, but don't let that cover up the fact that it does seem to be worse. When compared to the M8, it's all the more apparent. The M8 produced loud, full sounds with actual bass and separation of instruments in music. In contrast, the S5 sounds muddy, tinny, and basically dead.

So that's my poorly organized attempt at a comparison review. I tried to be unbiased when looking at the S5, but I walked away very happy in my decision to get the M8 just as I walked away from the S4 last year happy that I was getting the M7.