I guess it depends on your shooting style. If you do a lot of portraits, then you will quickly want to upgrade. A couple issues with the SB-400 are that it can NOT be used with CLS (Nikon's wireless system) and as stated before, the bounce is very limited, landscape style only. If your using it outdoors for a bit of fill it might be ok. and the SB-600 is now discontinued so you need to find a used one or move up to the Sb-700.
I would shoot with it for a while and see if it fits your needs,and then move forward depending on what you end up liking to shoot.