I have the Metz 50 AF-1 for my KX and really like it. It offers a lot for the money and if you every decide you want to go off-camera, it gives you that option. Using P-TTL is a good choice if you are going to be taking family shots, because it adjusts itself as the scenes and lighting changes.
As many of the reviews mentioned, the LCD menu is a little awkward but becomes easier to use the more you use it.
Here is a link to reviews if you want more info:
Metz mecablitz 50 AF-1 digital reviews - Pentax Camera Accessories: Database and Reviews