Arno is right. There seems to be a lot of confusion in the community with regards ESP and MS's material doesn't do much to explain things.
Basically, as it stands now with ESP v1.0, ESP is essentialy FSX Deluxe without the game wrapper front end, (for selecting flights, etc.) That's all.
The main differentiator between FSX and ESP is that ESP is licenced for commercial use, where as MS's agreement with third party content suppliers like Jeppeson only permits FSX to be used for home use. That's it.
So for FSX enthusiasts, ESP doesn't offer anything more (apart from a little more info in the SDK with regards air files) than FSX. I use ESP at work for commercial stuff, but at home, I wouldn't want it. FSX offers me more.
It's a mistake to think that because you may be an aspiring pilot or a real hardcore FSX enthusiast that you should want to use ESP. The aircraft physics and core functionality is identical, (some would say "unfortunately.")
ESP is designed so that commercial companies can take the core simulation and world engine and build a standalone bespoke simulator out of it. You could do exactly the same thing with FSX using exactly the same techniques, but it wouldn't be legal due to EULA restrictions if you wanted to make a commercial product out of it.
To be honest, ESP v1.0 is too limited in many ways because of its commonality with FSX. The APIs are far too restrictive to be of use to people used to coding flight sims using traditional high-end toolkits, (like VegaPrime or MetaVR.) Having said that, I believe ESP will likely find a new, more applicable market, when they expand its features to inclde sea surface and macro-level urban environment modelling and better land vehicle surface interaction.
In short, if you got ESP, (and you currently need a volume licensing agreement or MSDN subscription to do so), you'd be disappointed if you're expecting a different experience to FSX.
Si