For now, i'm waiting for .24 to come out before restarting a career, money will make think way more interesting. Right now, it is sand box experiment only for me. Well ... Sand box is good to try stuff without restriction (apart from DV of course). No need to think of science, (for the future) money, tech tree and dying kerbals. Just build and see if it work. it is also a good tool to prepare a mission for career, see if your design is actually working, if your mission will fail due to planing error... etc you can see it as a real scale simulation if you want.