In regards to the Miracle Berry Tablets - I have yet to try them myself, but I've met a few people who have and they swear by them. There was an OraBrush (tongue brush) commercial that demonstrated the effects once. People would come up, try the berry, and then bite into a lemon that would taste sweet. Then they would brush their tongue with the OraBrush, try the lemon again, and it would be sour. So I would agree with Dampmaskin on the whole idea that they mask sourness, but not bitterness.
As for making a bad eJuice taste better? Well, the only real way to do that is to mix something else with it - that is, if it's already had the time to steep and develop the full flavor. I wouldn't think that drinking water with MM mixed in would help. It might make the water taste good, but I don't see it staying on your tongue. That's why the Miracle Berries are tablets. You chew them which coats your tongue in them and causes anything you eat/drink for the next few minutes taste sweet until the coating on your tongue goes away.