So how fast is 4G LTE fast? Well 4G LTE has speeds ranging from 5MBit/sec to 12MBits (this can go higher or lower depending on network congestion), I have had my Lumia 900 for 3 days now and I average 10-12MBits/sec in internet speed. So just how fast? let's put this in contrast, your 3G network speed can go at a max 3MBits (that's the highest I have seen) and right now, I prefer to use my phone to surf the web than my home's WiFi connection, yes! it's that fast.
As you can see, there's a huge difference in speeds, the image below shows various speed tests on my Lumia 900, the first one (at the bottom) was when I had only 3G network, it records 3.28. while, the subsequent tests that followed was when I had finally activated my phone to LTE and as you can see there is a HUGE difference:
But not everyone will be able to use this 4G LTE blazing fast internet connection, you will have to be living in an area with LTE coverage. that's one of the shortcomings of LTE; Limited Coverage.
The Nokia Lumia 900 is an LTE enabled phone and if you live in an area without LTE coverage, you will not experience this blazing fast connection.
So how do you know if you are in an LTE coverage zone?
First, you can check At&t's LTE Coverage Map; It shows where LTE is available (indicated by Orange Dots), and you can further click on the orange dots to see a more detailed map of what cities/districts are covered.
If your area isn't on the map, don't worry, At&t has LTE coverage coming to different locations and you can even see area where LTE is "coming soon" (indicated by Orange Stars).
Of course, if you are not among the Orange Dots or Stars, you still have 4G (HSPA+) which has a slightly lower internet speed but it's way faster than "old 3G", normal 4G can go as high as 8MBits/sec. it's indicated by the "dark blue" color on the map and as you can see it has a wider coverage area than LTE.
Another way to check if you have LTE coverage, is on your phone. By just swiping from the top edge of the screen, this is the area that shows you your notifications like to show you battery life, WiFi, time, network signal, amongst other things.
The Network Signal is a quick indicator of whether you are on 3G, 4G or LTE. If yours doesn't say LTE, instead shows you 3G or 4G then you are not in an LTE coverage area. of course whenever you get to an LTE coverage area it automatically switches to LTE.
As you can see from the image below, it indicates I am on LTE network.