Yes I believe in Karma and it explains allot to me.
If bad people have so much prosperity it is because at one time they had good karma, then they got arrogant and are now sewing bad karma that has yet to catch up with them. Think of these financial managers who spend their life building up a good reputation, get greedy, and then ruin it all.
Or, some extremely good people live in terrible conditions and are unhealthy. Its because they are still reaping previous bad karma, but they have learned and are now sewing good karma that will eventually manifest as blessings in their life.
Truly blessed people have been sewing and reaping good karma for a long time, and all the bad karma has been exhausted. At the extreme of this you have Saints and Monks.
To me this explains all the mystery behind why "Life isn't fair". It actually is fair based on karma, we just can't see the entire picture, especially because karma is said to be spread over many lifetimes.
Karma is also why we should not compare ourselves to others and be envious. Our only business needs to be sewing good karma, and the blessings we want will eventually come.
For me I am the second case. I still am suffering from my previous anger, arrogance and irresponsibility.
However, now I follow the basic teaching of Buddha on a daily basis: don't do evil, do good and purify the mind.
Since doing that I am experiencing a stability like I have never had in my life and I am very optimistic, even though I am still experiencing difficulties from my previous actions.
So I think understanding Karma is very useful to calming my anxiety of the future. If you do good each day, avoid unethical behavior, and do things to quiet/straighten out your mind (meditation, TEA forms, exercise, etc.)... then tomorrow will take care of itself.