If you are not use to c++ or direct x xna could be a good start for building an engien. It seems easy to use and also user friendly. As you said it's look like a simplification of manager x or direct x, but it's seems to be a good start if you are planning to develop your own application. I mean if using a toolset is useless why solution like 3ds or torque exist ?
my point is even if I don't know this game api ( I just start looking at it) it's seems to be a good solution if you want to make your game or your engine, powerfull and easy to use, good documetation, nice feedback from the community. the possibility to export on xbox or xbox 360 ( even if it's just optional) could be a good alternative for young indies developpers or even for hobbist ( I planning to do an architecture visaliser)
