I don't have any of the problems with the Acceleration pack that the other reviewers describe. I can't help doubting the hardware they are using. The opposite really; the missions mostly work well, but best of all is that the game runs smoother than before- probably due to the inclusion of the service pack SP2. Racing is a tremendous addition to the game.
For me though, the SDK is the best part, modding the game is often more fun than "normal play". Even with the rather fiddly proceedures involved in making scenery, custom buildings and changing the landscape, the satisfaction of making and sharing mods makes up for it. There is an enormous network of support sites and highly skilled techies out there that are willing to help got you started.
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For reference: PIV E6600, 2Gb DDRII RAM, GF8800 GTS 640MB graphics. 24" Dell screen. This Sim really does need the newest computers to run well!