I have the new HTC One X. I do not have LTE coverage in my home area, but recently returned from a trip to San Diego, where I was able to try out LTE.
In areas with a strong LTE signal, it worked just fine. In areas with a weak signal, it would not connect to the Internet, and would not revert to the regualr 4G (HSPA+) service, making the phone unuseable for data. This is a serious problem. ATT should seek a software fix soon, or there will be a lot of unhappy One X owners.