The HTC One first launched online April 19th, but if you wanted to buy the phone directly from a physical store, you had to wait until…today! The HTC One is currently available for purchasing via T-Mobile stores nationwide. However, T-Mobile says the phone is only available from "select" stores, meaning you should probably call in advance before stopping by.
If you don't plan on buying the HTC One directly from HTC's website (which is great option if you want an unlocked phone that can work on any LTE network), T-Mobile's deal is your next best bet. You can snag the phone for a $99 down payment and then pay off the rest in monthly installments of $20 for 2 years. OR you can just buy the phone for $580 directly from T-Mobile. Bonus: you'll also get a free car dock, even without a two-year contract.
The benefits to T-Mobile are that you don't get locked into any sort of contract and you don't have to pay much up front. But you get a carrier-branded phone, so that's the trade-off. Find out about