Just around this time last year I wrote about 3G Roaming in the US with a Local SIM card that could be ordered from abroad before starting the trip. While it worked well, the main drawbacks were finding a mobile that would work on US frequency bands and the 'limitation' to UMTS. Also, the network kept dropping my VPN connection at random intervals. A year later, networks and offers have significantly advanced.
This time, I bought a T-Mobile US Prepaid SIM for Internet connectivity after arrival that would not only let me use their UMTS but also their LTE network. The cost for the SIM card was $15 and options that can be selected online range from $5 per day for 500 MB over $30 for 3GB for 30 days to $50 for 7 GB for 30 days. Not cheap but 'business traveler' affordable. Also, the SIM card is kept active for up to 365 days which is great if some time passes between trips to the US.
Speed wise I could easily reach data rates of 10-15 Mbit/s in downlink and 8 Mbit/s in uplink while my tethering device was camped on LTE band 4 (1700/2100 MHz) on a 10 MHz carrier at the hotel I stayed in Kansas City. I also noticed that another 5 MHz LTE carrier was on-air on band 2 (1900 MHz PCS). Reliability wise the network has also made a great step forward as I didn't notice a single VPN drop over the days.
Another thing that has significantly improved since last year is the availability of mobile devices sold in the EU that support some of the US LTE frequency bands. The iPhone supports a phenomenal 20 LTE bands and other devices, e.g. some from Sony, include support for LTE band 4 that is used by T-Mobile US and others. Here's an example from their German web presence. So if you travel to the US it's worth finding out which LTE bands are supported before you buy it.
All in all, the SIM has served me well and offers like this are another step in the right direction towards global affordable and fast Internet access.