I've rather enjoyed my play through the career mode, made me come up with some inventive designs etc and has even made what would have been mundane things in sandbox such as sending a probe to Duna a huge achievement. In sandbox I would have just send a manned mission straight to the red planet, but because you want to make sure you can bring back your kerbal, I first sent a probe out with the relevant scientific equipment and tested what will be my interplanetary prime mover for the manned mission, I know I could have done this in sandbox, but having limited parts available gives it that extra something.