It's not the extra aircraft and missions that make this add-on desirable. It is the update to FSX, which fixes many problems with the original, and adds features such as controllable turbo chargers, and Direct X 10 support. While DX10 support won't work so well with models made for DX9, it significantly increases framerates. I tested the DX10 interfaces at a heavy add-on airport, Manchester Xtreme, with my heaviest add-on model, PMDG 737NGX. With DX9 I was getting 6-9 fps, with DX10, this increased to 11-14. That is very siginficant. Normally, I have no problem getting 20fps with DX10 enabled.
Acceleration also includes an updated SDK, which is a great platform for developerss.
Buying this add-on will keep FSX alive at least until 2015 - may be longer. Therefore I'm recommending this add-on for existing FSX users 100%. Snap it up!