I have a Jupiter Melody, and also enjoy the water it produces.
There are two problems that can exist with these machines:
1. The water can be too pure (not enough minerals). These units actually require a mineral content in order to ionize the water; if the water is too pure then it wont really work well. But, I'm sure you could tell just from drinking the straight water if it was too pure ... probably not your problem.
2. The water can be too hard, or have other impurities. These units have filters on them, and will filter out a lot of impurities before ionization. However, often a pre-filter should be added just before the water enters the unit. If your unit is attached to the kitchen faucet, you might want to get a pre-filter for around $80 or so that attaches just under the sink. This should go a long way towards improving the water that comes out of the ionizer.