As of v.4.3 VirtualBox will have built-in video capturing support which allows one to record the guest OS easily through the VirtualBox Manager GUI. This feature was previously available but only by use of external programs (ffpmeg) and it required additional configuration. With v.4.3 (currently in Beta2) the screen capturing option is available within the VM window (under Devices–>Video Capture) or from the VM settings (under Display–>Video Capture). The latter option allows customization (resolution, frame rate, quality) that can be made individually for each VM.
This is a sample video I created at high quality settings (1875kbps) with 1600×900@25fps. The video lost a bit in quality due to YouTube’s automatic processing (I didn’t optimize it in any way) but it still looks reasonably good in my opinion.
VirtualBox 4.3 also brings a number of improvements and additions which you can read about in the Beta1 and Beta2 release notes.