These devices sport a heavily forked version of Android (Jelly Bean, or 4.1.2) on the X series. 4.1.2 was released back in October of 2012, and thus shows how long Nokia has been working on this series of smartphones. The X features hardware very similar to the Lumia 520, and even cut them some, making it a low-end model. The X, for example, comes with 4GB of internal storage (microSD is supported) vs. the 520's 8GB. Both sport a relatively minimal 512MB of RAM.
Aside from that, most of the changes are external, with a lack of Windows Phone’s three capacitive buttons and a slight camera change. The Nokia X series supports dual SIMs, too, something very important in the emerging markets being targeted by Nokia.
The X+ sports more RAM (768MB) and is slightly larger. Meanwhile, the XL is much larger, with a 5-inch screen (vs. the other models' 4-inches) and a bigger battery. All of them sport modest 800 x 480 screen resolutions. The XL has a 5MP rear-facing camera while the X and X+ only have 3MP cameras.
As heavily forked as the Android version on the X series is, there is no access to the Play Store (think Kindle Fire). However, it does support sideloading, and unlike Amazon.com Nokia will not go out of its way to block Google apps like Gmail, Maps, etc.
That being said, Nokia made sure to put a Windows Phone-like UI on the devices, with a Live Tiles type of interface.
Price- and shipping-wise, the X is available immediately at EUR 89 ($122). The Nokia X+ and Nokia XL are expected to roll out in these markets starting in the early second quarter, priced at EUR 99 ($136) and EUR 109 ($150), respectively. These prices are unlocked and unsubsidized, meaning no carrier contract is required.
The one big negative is a lack of LTE support, but considering the markets Nokia is looking at, it is probably not as huge a missing feature as lack of dual SIM support would be.
It's unclear how satisfactory the user experience will be, though. Nokia's choice of 4.1.2, probably forced by timing of product development, means that it's missing the recently introduced modifications made in KitKat, which lower the HW bar for devices.