Its an ok flash, just dont expect miricles from it. I've owned one and sold it after a few months. Mainly because it only bounced up, you cant bounce to the side or behind you so I out grew it fast!
It isn't the most powerful flash on earth either, much better than the built in one though. It also has no controls so making flash power adjustments is a bit long winded as you have to do it in camera.
I would seriously think about the Sb600 instead, it has much more to offer.
I guess it depends what you want. If it really is just snapshots and you dont intend to take you photography much further then you will find the sb400 an improvement over the on board flash. Hope this helps.