This is very rewarding if you love watching liftoffs.
There most certainly are missions! With all the various addons you can recreate almost any mission NASA has done. My favorites are project Gemini missions complete with a spacewalk.
If all you care about is kills and scoring points, this sim is not for you. But if you are thrilled by MS Flight Simulator where you just tool around and enjoy the ride in almost any airplane that ever flew, this ride will blow you away.
There are even some science fiction addons such as flying an Eagle from Space:1999 over Moonbase Alpha.
Of course, once you have seen everything you like and made cool desktop images from screenshots, you can go back to combat flying

I basically install and play this sim once a year to see all the progress that has been made.