Apple has finally begun selling factory-unlocked iPhone 5 smartphones in the US. As with previous releases, the move comes roughly two month after carrier locked versions went on sale in the country, whereas Canada and some European nations see both versions launch alogside each other. The unlocked phone starts at $649 for 16GB of storage, with the 32GB and 64GB models adding an extra $100 and $200, respectively.

The device is being sold through the company's online store, with ship-by times of one week and orders limited to two handsets per customer. 9to5Mac says retail stores will start selling off-contract phones in the coming days, although unnoficially some stores were already doing this as early as last week, one source tells us.

The phone is great for frequent travelers who would like to pop-in SIM form a local carrier while abroad to avoid roaming charges, or anyone looking to avoid multi-year contracts and expensive plans by paying more upfront. It should be noted the unlocked iPhone 5 is GSM only, which supports AT&T and T-Mobile in the US, and will not work on CDMA bands. There's also LTE support where available but Apple doesn't specify which bands this iPhone covers -- based on a core part number Engadget says it'll only work on AT&T and Canadian carriers.

For a better bang-for-the-buck deal on an unlocked phone the Nexus 4 is a solid choice -- if you can find one. It's only $300 or $350 for the 8GB or 16GB model, and its specs are about on par with today's top performing Android phones except for the lack of LTE and microSD expansion. Google is having a hard time keeping up with demand, however, witht the pre-order page listing 8 - 9 week ship times for the phone.