The OnePlus X is actually a really good phone, made better by the fact that it has a great price tag of just $249. A glance at the phone's spec sheet gives you hope for this compact phone that's wrapped in metal and glass — that is, until you look at the supported radio bands.
|GSM||850, 900, 1800, 1900 MHz|
|WCDMA||1, 2, 4, 5, 8|
|LTE||1, 2, 4, 5, 7, 8|
Now that's the radio listing for the "North America" model (there's a separate EU/India model), which presumably means it should work for users of the largest GSM carrier in the U.S., AT&T. Except that the OnePlus X is missing one LTE frequency, Band 17, that AT&T runs a vast majority of its LTE network on. Yes you can still find Band 2, 4 and 5 in use in many markets — including those where Band 17 is the primary frequency used — but if you were to fire up a phone with an AT&T SIM in it anywhere around the country you're most likely going to connect to Band 17.
So what happens when you use an AT&T SIM — or remember, any prepaid MVNO operating on AT&T — in your OnePlus X? Well, you're most likely going to be on HSPA+ data. AT&T's LTE network footprint is large, so large in fact that you probably haven't seen HSPA+ in some time, and while speeds there are usable they definitely aren't LTE speeds. Even though you will find some LTE operating on other bands (albeit likely at slower speeds) as you move about, you shouldn't count on it — and that gets really frustrating, really fast.
Now AT&T is pretty much the only carrier in North America actively using Band 17, save for Rogers in Canada which supplements coverage in some areas with it. Having Bands 2, 4, 5 and 8 in a phone covers any major GSM carrier in North America, and each radio band you put in a phone does cost money. But that doesn't excuse leaving out the primary radio band in use by the largest GSM carrier — with some of the most popular MVNOs, too — in the region the phone was made for. In any way. And it unfortunately means that anyone on AT&T or its MVNOs shouldn't consider using a OnePlus X.
While we're talking radio frequencies, eagle-eyed enthusiasts will also call out the OnePlus X's lack of Band 12 LTE support. While we agree that's important for the future of being compatible with T-Mobile's network, it's not quite the stumble that missing Band 17 for AT&T is, and there are lots of very capable phones out there today that aren't including (or aren't certified for) Band 12 with T-Mobile just yet. As far as T-Mo goes, Band 2 and 4 are the standard ones you need, and they're here.
AT&T is another story, and it's not a fun one.