Once again, Windows phones are going to suffer limited availability and carrier compatibility.
The last flagship Windows Phone available in the US was the Lumia Icon (known around the rest of the world as the Lumia 930). It was exclusive to Verizon. When Verizon dropped it, there was no replacement high-end Windows Phone for Verizon customers, nor any widely available high-end Windows Phone in the US regardless of carrier. Before that, the flagship was the Lumia 1520. It was exclusive to AT&T. When AT&T dropped it, there was no replacement high-end Windows Phone for AT&T customers, nor any widely available high-end Windows Phone in the US regardless of carrier. Before that, the flagship was the Lumia 920. It was exclusive to AT&T. Same story.
Windows Phone has not been as successful in the US as it has in some other markets—parts of Europe, for example—but those fans in the US have continually been frustrated by the way that Microsoft (and Nokia before it) has made its handsets available. Microsoft has negotiated exclusive agreements to give one carrier a particularly desirable phone, and those carriers have responded by promoting those phones for a period of time before dropping them entirely. This was compounded by the cancellation of the "McLaren" that was meant to be a new flagship phone available in 2014.
What did it mean in practice? A person who bought the Lumia 920 when it was released on AT&T about 3 years ago who wanted to upgrade after 2 years had nothing to upgrade to. The Lumia 930 would have been a decent upgrade, except it was never sold in American markets with American frequencies. The Icon was a CDMA phone, built for Verizon's network (and, potentially, Sprint's). Lumia 930 owners now on the lookout for an upgrade are similarly out of luck.
The new Lumia 950 and 950 XL pair seem like pretty decent phones. The Windows Hello iris scanning authentication is novel, and the Windows Continuum capability could find some customer appeal. They're fast, first impressions of their cameras are positive, and for those people who are interested in Windows Phone, they're compelling purchases.
It was, therefore, a little disappointing when it was initially reported on CNET that both phones would be exclusive to AT&T. AT&T subsequently announced that it would be supporting the 950, but made no mention of the 950 XL. The Microsoft Store, by contrast, included a listing for the unlocked 950 XL, but had no information at all about the 950.
It subsequently turned out that the initial CNET report was wrong. AT&T will be the only US carrier to sell the phones, but it has no exclusivity. The 950 will apparently turn up on the Microsoft Store—eventually—and both the 950 and the 950 XL should work on AT&T and T-Mobile. Availability in other countries is as yet unknown, with the exception of Germany. In that country, Deutsche Telekom will distribute both.
But while this situation is better than what was originally feared, with T-Mobile and AT&T customers having both phones available as an option, it's still far from ideal. The lack of in-store representation—and the carrier financing agreement that goes with it—will limit availability. T-Mobile's outspoken CEO John Legere has said in a handful of tweets that the fault lies entirely with Microsoft. He says that Microsoft made the decision and that T-Mobile didn't turn down the phones. They were never even offered.