I love my Galaxy S4. I purchased it on T-Mobile, and was pleasantly surprised to find their 4G LTE coverage nearly everywhere across Seattle despite it not being officially online yet.
The S4 blows away the performance of any phone currently out there, and the software is well-designed. It came with a lot of bells-and-whistles enabled, which I promptly disabled in the name of simplicity. This is the strength of the Android experience- you are able to tailor the phone's interface to exactly what you want quite easily. Some of the Samsung apps were a bit annoying and duplicated existing more-streamlined Android features, but I had everything tweaked the way I wanted it within a day of use. The phone is only slightly bigger than my previous phone (Nexus S 4G) with an extra .25" in width and .5 inch in length, and is slimmer and slightly lighter. The increase in size is noticeable, but not onerous. The additional screen resolution is helpful and the display is beautiful. Yesterday I noticed an attribute that I've never found in a phone before (which should be appreciated by fellow ExPo'ers)- the display was viewable in direct bright sunlight while wearing polarized sunglasses (those are usually incompatible). I personally appreciate the plastic back case. The front is smooth glass and it has a metal band around the outside, so it is only the back cover that is plastic. It enhances durability, and it easily replaceable if you ding it up (try that with a shattered iPhone). The plastic case also makes it easy to replace and provides good protection for the battery, SIM card, and MicroSD card (yes, you can easily and cheaply add 64GB of storage).
For the balance of features, performance, durability, and quality network that I'm looking for, I am very happy with my purchase. Today, my week is done and the phone gets factory reset and handed over to my girlfriend so she can trial it for a week of hospital resident use. I've already decided I'm keeping it afterwards, but now we need to know how many of them we want.