Hi, I am a little confused about direct X and its impact on FSX, I saw some people that flight with the direct X option un-checked and others with, what's the difference? Which is better? Depends on what? Thanks

I assume you are referring to DX10.

 

I personally like its performance improvements (especially in exterior views). However, some do not experience performance improvements that justify DX10's incompatibility issues, which cause some payware airports to glitch graphically. 

 

I recommend reading the DX10 Forum documents, backing up FSX.cfg, and experimenting with DX10 yourself.