During launch week, you could buy an unlocked Samsung Galaxy S7 for $599, a hell of a deal at the time. Here we are a month later and that same phone can now be had for $559. At most US carriers, you are looking at close to $700 for this phone, so you can see right there that this phone is being sold at a substantial discount.
This is the model (SM-G930F) that I own and use on a daily basis. It’s an international unit that runs an Exynos octa-core, which most across the internet would agree crushes the Snapdragon 820 variant, both in terms of performance and battery life. It’s the version that we wish Samsung would have released in the US. Other than the processor switch, this is the same great Galaxy S7 that you know.
This particular unit will work without a hiccup on T-Mobile or AT&T and is a GSM phone (no Verizon or Sprint here).
As for downsides, just know that Samsung Pay is not currently supported on these international units. Samsung seems to have blocked Samsung Pay by model number, from what I can tell, rather than by location or SIM. These units may get access to Samsung Pay once it launches in select countries later this year (like the UK), but there is no guarantee.