Hopefully it'll somehow allow FSX to work on my computer again...
Is it worth it? Will it cost a lot less? Are there really enough bonuses to make it worthwhile?
Well, not really. The expansion pack, even if it icludes the DX10 patch, won't enable you to play FSX again. In fact I really think that FSX is going to get really slower in DX10 and it won't look much better than in DX9. And only people who have an 8800GTS/GTX/Ultra or an HD2900XT will be able to play it in DX10. Anyone who has a card inferior to these (even if it supports DX10) will probably not be able to play FSX decently in DX10.
The thing is that manufacturers say that cards are DX10 compatible but that only means that they can execute the DX10 instructions, it doesn't mean that the card can handle the DX10 games at decent FPS. We have seen the high end DX10 cards struggle to play the newest DX10 games.... and those games don't even look that much better than in DX9. When DX10 games start getting more and more complex with better visual effects, then all this will be even more accurate. No card under the 8800 series or the 2900 series will be able to run DX10 games decently.
This is really bad because a lot of people bought 8600's and HD2600's thinking they would be able to play the new DX10 games like Crysis, Bio Shock, FSX (patched), Lost Planet, Company of Heroes (Patched), Call of Juarez, etc. I truly believe they are wrong! I don't think any of those DX10 games (or any of the newer and better looking ones) will run decently under an 8600 or an HD2600.