The reason that the legal limit for drinking as been set at a blood alcohol concentration of .08 is that alcohol impacts you differently as your BAC climbs. This does not mean that you won't feel the impact at all if it's below .08, however. According to the Center for Disease Control and Prevention, relaxation, loss of judgement and a slight impairment of your ability to follow moving objects can start at just .02.
As you get up to .05, you can start to feel the effects more drastically, and you may even lose just a little bit of muscle control. You also won't be as alert, your judgment may be altered, and you could begin to have trouble steering. You'll still feel pretty good at this point.