Whilst I'm using a DSLR and removable Speed-light which are completely controllable via IR-remote, there are occasions where I may have to resort to my trusty pocket Kodak and its built-in flash or the pop-up flash on the Nikon.
It is important therefore to familiarise myself with its capabilities and restrictions.

Using full-flash will only be correct for a certain distance from the lens subjects closer will appear over exposed and the background will quickly become dark (inverse Square Law). The TTL function on the built-in flash does quite a good job of balancing out the light levels.
Shiny surfaces will look awful with the light source undiffused and directly from the front.
Shadow edges will be hard, contrast harsh.
No modelling in the lighting, very flat light.
Increased chance of Red-Eye.

All in all I was pleasantly surprised by the quality of the images resulting from using only the pop-up flash of the D90, this has a lot to do with the very clever software involved in the TTL mode, even in the little built-in flash unit. The position of the light source however, directly above and adjacent to the lens, is less desirable. These images are all taken with the 50mm f1,8 lens. Anything “wider” throws a shadow of the lens-hood onto the subject.