It's the fastest data network and it doesn't cost extra. If you have AT&T, you'll notice it drop to Edge or 4G HSPA+ when you leave an LTE area. LTE coverage is typically just in bigger cities right now.
Speed depends on where you are. I've had download speeds as high as 59 MB per second if I am near large metropolitan areas with LTE coverage. Conversely, I'm lucky to get download speeds much higher than 5 Mb per second in a rural 4G coverage area with three bars of signal. There are also conditions when LTE may actually be slower than 4G (good 4G coverage and poor LTE coverage at a single location for instance).