AnandTech's usual in-depth review.
Overall the Galaxy S 5 is a solid replacement to the GS4 (and definitely to any previous Samsung device). I find that pretty much all the flagships offer some set of tradeoffs that prevent any one from being the perfect device (iPhone's screen size, GS5's materials, M8's camera). It's unfortunate because I'd really like to crown a single device the king of them all, but instead we're faced with a handful of differing optimization points. Samsung got it almost perfect with the GS5. With a metal body, a rear facing camera with larger pixels (perhaps with some tweaks to camera output processing), a better NAND controller, and stereo front facing speakers, the GS5 would probably be perfect.
As much as I dislike Samsung - they simply have no taste - the fact of the matter is that reviews of their flagships are virtually always positive, and users have clearly voted with their wallets. Apparently I belong to a minority.