SIM-free means, in part, that you won’t get a carrier SIM card. You’ll still need to get one. Keep that (to me, a tiny bit of a hassle) in mind.
UPDATE: From the comments:
You can get a SIM card for free from T-Mobile, and it takes seconds to install.
Meanwhile, that SIM-free iPhone—unlike the GSM models— is fully compatible with all networks including CDMA. More importantly, it has the Qualcomm chip, which means it will provide about 30% better wireless performance than Intel-equipped models in terms of data speeds, signal reception, voice quality, and battery life. And due to all of the above advantages, the SIM-free model will also provide a significantly higher resale value.
This is the iPhone variant everyone should buy.
UPDATE 2: Also from the comments:
Dave: it may also be worth mentioning that this is a great option if you already have a nano SIM card in your existing iPhone.