Firstly the phone
Coming from a Galaxy S3 and a Nexus 7 (2013) I am suitably impressed with the Galaxy S4, now this isn't the standard S4 you will probably buy over the counter, this is the i9506 model, one of a handful of models in NZ that support CAT4 LTE (Xperia Z1, Note3, Nexus5, um... that's all I can think of off the top of my head). For $100 more you also get the Snapdragon800 CPU that is featured in the Galaxy Note3 and is significantly faster than the older model G4's (i9500, i9505, i9505G).
I know benchmarking is a stab in the dark at best but going by the XDA G4 speed test thread using Antutu, a i9500-i9505 can expect to get a result of around 24,000-27,000, the i9506 consistently scores over 33,000 (that is about 30% faster!).
So if you are considering a G4 make sure you ask for the CAT4 version, it is worth it.
4G network
Of course the first thing I had to do was to run the Speedtest.net app to test my speed from my desk at work in Christchurch and here is the result, this is faster than a home VDSL connection, about on par with entry level fibre. Bear in mind that CAT4 can go much faster than this and I will try again from home and Auckland over the next couple of weeks to see where some of the faster areas are (I have also been told that Vodafone is in the middle of upgrading to CAT4 at the moment and I should see that speed double very soon).
I use tethering quite a lot with my Nexus7 and laptop while away for work and I have a months worth of 4G data to use in the next couple of days so I will be really pushing the data through to see how it goes, I only have tethered 3G usage to compare to so I am expecting some nice results.
I also had my first HD voice call not long after the connection came on and had just been on another call on my S3 and the difference is amazing, its hard to explain just how clear it is, sort of like going from audio on a cassette tape to listening to a CD for the first time. God, that shows my age....
So is 4G worth it? Hell yes I say! Make sure you get a CAT4 phone to go with it for extra win!