I had the the S3 and the S4 so I decided to pass on the S5.
The thing I noticed about Samsung is that they make devices that come great out of the box packed with features that will be turned off after a few days of using the device.
I'm not a fan of the lag the Sammy smartphones seem to have either.
I think touchwiz needs to be toned down a bit as well.
If you live or are around rural areas a lot the antenna is an issue well.
Both my Galaxy S3 and S4 use to stay dropping calls when I visited my mom out in the country.
With with that being said the Galaxy S series is one of the top 2 or 3 smartphones on the market and I prefer their version of Android over many other platforms such as iOS.
I'll probably own other Samsung smartphones in the future but they need to tone down touchwiz, their feature heavy operating system that uses up RAM and internal storage, and invest in putting better antennas in their devices.