I can't stand using my wife's S3. I just don't like the way samsung does their software and I'm not a fan of an all plastic smartphone. I've been on a Droid Razr Maxx HD for the last year and a half and I've really liked it. I've never put any case or screen protector on it and though the back and edges have a few nicks from dropping it multiple times, there's not so much as a scratch on the screen.
Perhaps when the times comes I'll upgrade to a non Samsung phone. The only two Android phones I've used are the S1 and S3, so I really don't know what all Samsung does or what would be different about a vanilla phone. I was not planning on buying an S5 just to see the difference, but it constantly gets such great reviews it would be hard to risk something else when I'm happy enough with my current phone and the S5 is just a better version of it.
One annoyance I have though is the provider crap. My S1 was through ATT and had ATT apps that you couldn't delete. My S3 through Verizon is the say way, Verizon apps I'll never use but can't delete.
I see the appeal of buying a generic, non provider specific phone and using that, but for the $400+ price difference I'll just ignore a few icons.