Service should expand in the "coming weeks" to include parts of Chicago, Los Angeles, New York City, Phoenix and Washington, DC. You can also expect the Galaxy S10 5G to arrive sometime in the summer.
As with deployments at other carriers, there are going to be plenty of caveats. Coverage is still likely to be spotty, especially indoors where high-frequency 5G often can't penetrate. And whatever you think of the first 5G devices, it remains true that your options will expand significantly in the months ahead. Early 5G service is still focused primarily on bragging rights, and it could take a long while before it's as polished as LTE.
The rollout leaves just one major US carrier left without 5G: Sprint's would-be partner, T-Mobile. It's expected to debut faster service in the second half of the year. Provided that happens in a timely fashion, though, it could be just a matter of months before it's a question of which 5G network suits your needs, rather than whether you have a 5G option in the first place.