shure its possible.
i planning doing this way for my games.
BSP.:
PRO
: There are many bsp levels spread over the internet
and its fun sometimes to play your game in the same enviroment as you did with HL (for example) befor.
You can Load the entire world with one comand and its easy to setup cameras and playercollisions.
Contra
: You can only use Rectangular shapes.
.X :
Pro
: Full freedom to create any shape u want.
Contra
: Have to write your own LoadFunction.
You have to take care for collisions. You have to find a way
to import leveldata (object positions and properties.)
So its up to U.
You have more freedom in leveldesing with .X files.
but have to care for more thimgs in programming.