Our engine of choice is Unity 3D and luckily enough, thanks to the great folks at Oculus, it has day 1 development support for the Rift. Not just token support either, this stuff is robust and documented. You can download the OVR library from the Oculus dev portal, and it plugs into Unity as easily as any Unity Package, a double-click and you're away.
Oculus provides prefabs for their two-camera setup, including one with a basic character controller. Adding basic Rift support to your games is as easy as choosing which prefab to add.
Unfortunately the Unity preview window can't go full-screen, so unless you like headaches, testing your game with the Rift requires a quick build. The Oculus plugin provides a button to make this an express procedure.
There are a few issues to be aware of regardless of the game you make; camera clipping through your avatar's shoulders, the low resolution making distant detail and text difficult to read, and rendering everything twice from two cameras. Like any hurdles in game development these can be addressed with time and effort.
Overall the process is essentially painless. Kudos to Oculus and Unity Technologies for this great integration.
Join us next week for our experiences playing a few Rift-ready games and some predictions for where VR goes from here.