You will also receive all of their weaknesses.
So if you were to pick superman's abilities, the government could depower you using red solar radiation. If you had Martian Manhunter's abilities, you would be weak to fire. If you had the Hulk's powers, you would turn into a mindless rage monster. If you had Thor's powers, you would be slower than a 9 year old.
There is no way to get rid of the powers or gain more power, such as going into the sun to super charge or turning into a burning martian.