I would fully support the upgrade from the Galaxy S4 to the S5.
Had my old S4 for about a year and a half and I was very happy with it. Only reason I decided to switch was the battery was starting to fail as I put a lot of hours into the device. But once I did, I'm very happy with the decision. I've had my S5 for about 5 months now.
Just a few important things I've noticed that are acctually relevant (to me);
Pros:
Battery life is much better. The milli amp hours are not only increased on the battery, but the device does not has as many background processes I guess. Difficult for me to put a hard number on the improvement, but I'd say about 25% increase on a very heavy use day. The ultra power saving mode and regular power saving mode are a drastic help. Any time you are just reading a Web page or listening to music on your phone, throw on the regular power saving mode and extends the life. Even more so in the ultra saving mode when I know I'm going to just have it in my pocket unused for an extended period of time (example: while I'm at work or while I'm seeing a movie.)
Processor Speed: To say the S4 was a great phone is easy. To say it was perfect, not so much. It's largest draw back in my eyes was the fact that it would have issues with freezing up or not responding in some way such as the touch screen not working or the power button not putting it to sleep. All that is caused by the S4s deficit of processor speed that is required for the apps that are available. Running all the bells and whistles, the S5 really handles all I can throw at it without having that bogged down, sluggish feeling. Occasionally the keyboard will delay a few seconds from what I'm typing to acctual input, but I'm talking like once a week. Maybe.
Screen Perfection: Straight out my favorite thing about my S5 is the vibrancy of the screen. I've always loved personalizing my phone. It's only made better with the quality of the S5 color and sharpness. Besides the pixels on the screen, as far as the glass is concerned, it's amazing. I have the Samsung flip cover now, but before I had that, I kept the phone in my pocket without anything covering the screen at all. Just the bare glasses. Not a single screen, buff, or crack. Looks like the day I unboxed it. I would have gone caseless , but the phone was a little too thin to hold for my hands at that point.
Fast charging: The USB 3 connector is the weirdest feeling thing to try to plug into the device, but it is really worth it. If the phone is turned off when I plug it into the wall, I'll go from 0% too 100% before I've realized. I believe it takes about an hour. Never really sat and watched it. But I know usually when I go to check it, it's either fully charged or very high and every time I think "Wow, can't believe it's already that charged"
Waterproofing: Another reason the S5 is funny to plug in is there is a little flap that they had to place over the charging port in order to plug up the hole when you arnt using it. I know a few people that have pulled this flap off, but I think that's just crazy. The waterproof capabilities of the S5 make it perfect to bring your phone into a relaxing pool trip or something. I love the capability to bring my phone out in the pouring rain and not worry about missing a call because I'm worried it will damage my phone.
Cons:
Overheating. This happens with every phone I've had so I really can't say it's just an issue with the S5, but it still hasn't really been fixed, so it's worth addressing in my opinion. I really does take a long time for the screen to warm up, but when it does it can really be uncomfortable. The phone has an auto cool down sequence that will trigger when the phone reaches a certain temperature, but when this happens, part of the cycle is to really nerf the processor until the phone cools off. During this time, the phone is really slow. Too slow for me to use anyway. Just don't run a million background apps and youll be fine.
The home button. Seems like this wouldn't really be problematic, but for me it almost was a deal breaker for the S5. The home button has no way to determine if it's being pressed while it's in your pocket or something. So all the time while I was walking around with it sitting in my pocket I could feel it vibrating like the screen was bring touched and I know it was messing up again. The idea that the phone can be unlocked at all while it's in your pocket at all seems strange to me. The phone has a proximity sensor (example: when you hold your phone up to your ear when your on a call, it shuts the screen off so you don't press buttons with your ear. You'd think it would implement the same idea in android to stop your pocket from unlocking your phone. They didn't. My remedy was the Sview cover. It physically covers the home button and not it's not an issue. But if you don't like the idea or feel of the Sview cover, this might be problematic for you.
The heart rate sensor. Never seems to get a read of my finger. Even if it did, I don't think I'd want to open an app to check my pulse in the middle of an activity, just to track something like that.
Okay so that last one wasn't as much a con. Just feel like it was worth bringing up.
Anyways hope that helps.
she is in the middle somewhere, just had cataract surgery.