Everybody else commented on the superb build quality and all that. And yeah, it's nice. Looks good, the sides are nicely rounded with just a bit of an edge, so it's not completely slippery. It's not as nicely built as the M9 or a Sony Z3 - or the iPhone 6 it so blatantly tries to imitate, for that matter, but it's easily the nicest phone Samsung ever built.
Everybody also commented on the lack of a removable battery, external sd card and waterproofing. Waterproofing is very useful in a country like mine where it rains more often than not, so I'd like to have seen that. I've never changed a battery in any previous phone, though. Ever. And using an external sdcard has been a hobbled experience for the last few major Android versions. So yeah. I mean, to each their own.
The camera is awesome. It opens really quickly when doubletapping the physical home key, focus is fast and accurate and the photos are gorgeous. It's a very competent point-and-shoot solution, and there are tons of special "pro" features if you want to get creative with your shots. I'm not sure if this camera is better than the recent Sonys but I'd say the image processing is better. The S6 produces consistently good results. Totally approved.
The screen is nice too, but I'm not as blown away as other reviews I've read. My device is not as color-accurate as I've come to expect from the praise of others - in fact it has a slightly greenish tint. I hate to sound like a Sony commercial, but again, other top shelf phones have had very nice looking screens too, albeit at 1080p, but to be honest: I can't tell 1440p from 1080p on a 5.1" screen in any real world scenario. The 1440p spec is functionally useless, and comes at the expense of battery performance.
Because oh dear. The battery on this thing is wildly inconsistent. I am getting around 14 hours of total battery life after unplugging with anywhere between 3 and 4½ hours of the rather more interesting screen-time. So the truth is somewhere between "horrible" and "not bad, I suppose", which I'd be less than happy with if I'd built myself a flagship. Coming from the Sony Xperia Z2/3, I'm used to seeing at least 5-6 hours of screen-time and charging every 2-3 days.
Some would interject that having adaptive fast charge eliminates this issue. Which is only a half truth and utterly beside the point. While being able to add a substantial charge in minutes is extremely useful, there's nothing like just stuffing a charged phone in your pocket in the morning and going about your day without a care in the world or a charger in your other pocket, knowing it will most certainly last.
The S6 doesn't last and in comparison to my Sony Z3 and Z2, using the Galaxy S6 feels like watching one of those treasure hunter diver movies, where you involuntarily hold your breath while the protagonist tries to make it to the surface without her oxygen tank. Mmmmm, Jessica Alba.
And speaking of pretty... No Samsung review from me without mentioning touchwiz. Touchwiz has long been an atrocious, battery depleting, resource hogging eyesore of useless bloat, gimmicks and glitter. It still is, and all the shit is there. It's mostly hidden away, and most of it is disabled by default, which I suppose is progress. The S6 does, however, come with a generous offering of preinstalled apps, most of which cannot be uninstalled. Some of them can be "disabled"... I've never really understood what that would accomplish, as they seem to zombie back to life every time Google Play autoupdates.
If at least I could have S Health not waste battery on monitoring my every step, I'd be happy. That's my main gripe with any intrusive OEM overlay - they lack an "I don't give a shit" function. It's an archaic practice. No means no.
All in all though, Touchwiz doesn't really intrude on my daily use, and doesn't seem to slow the phone down. The phone is really fast and fluid, and I haven't made it drop a beat at anything except browsing various news sources in Google Chrome. I suspect that's more of a Chrome issue. Everything looks nice enough and the fingerprint reader works well.
Here's a thought for you, though. If you grab the latest and greatest phones from Google, Sony or Motorola, you largely get the naked Android experience, which has been pretty good since the introduction of Android 5 late 2014.
If you're coming from that to the Galaxy S6, you're in for an experience so utterly alien that you might as well pick up an iPhone. With the iPhone, you'd also be getting a very alien user experience, but you'd be having it on prettier hardware. Lighter, thinner, slightly smaller (or larger (6plus)) with an even better camera and most certainly better battery life.
If I had to make a TL;DR conclusion of sorts, I'd say the S6 is rather good, but also slightly disappointing. It's fast, pretty, well built and takes great photos. The UI is alright and mostly convenient, and the screen is...nice. Everything comes at the expense of battery life. It charges quickly and wirelessly and all that, but for convenience there's no match for not having to lug a charger around at all. It's also rather expensive.
Ultimately I can't shake the feeling that the Galaxy S6 tries too hard to be an iPhone, and it's not nearly as good at being an iPhone as the iPhone is.