I've had mine for 5 days and I'm really happy. I came from a Droid Bionic so you can imagine how big an upgrade this is for me. I always avoided Samsung phones for two reasons, first was the creaky, flimsy plastic build, and the second was the gawdawful garish, laggy Touchwiz. I'm happy to say that the build quality of the S5 is head and shoulders above their last two phones and Touchwiz, while still mildly annoying, is smooth and usable. Mine has no lag, the camera takes great pictures both outdoors and indoors under decent lighting (this is not the best camera for darker indoor lighting such as in a bar or nightclub) and the battery lasts all day.
I'm self employed and I work in my customers homes and offices and I use the phone to navigate and play podcasts, and I usually use an app like Triplog to record my mileage at the same time. I HAD to have the Bionic plugged in, even with the larger factory battery or it would be dead by 3pm. And, even with a 2A car charger the battery would not charge while doing all that at once, and I have two of the factory extended batteries. The S5 will do all that without being plugged in and I'll still have 40% battery life when I get home. And if I leave the house and forget to charge it overnight, it will charge while doing all that at the same time. If I start out with a full battery and keep it on the charger in the car while navigating, I'll get home with the battery at 80% or above.
The camera takes super fast, super sharp pictures that can be zoomed in on and cropped. I use it both for fun and for documentation purposes. I never have to wait or kill other apps to get to the camera and get a quick, quality shot. I don't take low light photos, so this camera is perfect for me. It also does great with pics to PDF and for mobile banking deposits. I don't have one bad thing to say about it. My wife upgraded her Bionic to the S5 also, and she's a real estate agent and is now leaving her 7 MP Canon camera at home, the first house pictures she too with the S5 turned out great. And she's still experimenting with the Virtual Tour mode.
Honestly, we loved our Droid Bionics, we had 3 in the family and until the upgrade to JB they were great. JB never ran right on them and I'd finally had enough. I skipped on the phone that I really wanted, the MotoX because of the sub par camera. I agonized over the M8 vs. S5 as bad as Sheldon Cooper agonized over the PS4 vs. XBO. But in the end the camera and water resistance (the washing machine video really pushed me over) of the S5 sold me. I would have loved to have had the software of the MotoX, the build of the M8 and the camera and IP67 of the Samsung, but in the end the S5 won.