Besides the fact that DX10 aint compatible with most add-ons/vice versa if you like, it still remains unplayable.
Lol I once said it here on a thread and some poster with 10< posts told me I dont know what I talk about. That most add-ons work with DX10 and that it enhances the FSX experience with leaps.
I havent tried 8.6 since it is only BETA but DX9 with 8.5 is great.

8.6 is really out? WOW, why go back to 8.5? How is 8.6 in DX9?

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