Well, here's how it works. An electric current is run through the water as it passes through the "machine" and it is also subsequently filtered. Water has very strong intermolecular forces, which means that each individual water molecule will "stick", electrostatically (almost like a magnet), to another water molecule. So your body, instead of viewing one small water molecule, will "see" maybe 60 of them stuck together and treat it rather as one very large molecule. This makes absorption less efficient. So when the electric current is passed through, it desorbs the electronic charge between molecules and liberates them. Additionally, a very large negative charge will form on the oxygen atom of the water molecule and a positive on the hydrogens, and there is a large dipole (unbalanced electric charge) on the oxygen atom. This strong negative charge makes the molecule slightly alkaline or basic. This is said to aid in absorption, because the potential between the molecule and stomach acid is now greater.
In other words, it chops up the water molecules away from each other so that there are more, smaller "molecules" for your body to absorb and it makes it slightly alkaline. Is it worth it? I don't know, I've never tried it really. I don't know if they make any that you would consider "cheap." My best advice is that if you do have the money, it couldn't hurt. Remember there is probably some sort of cartridge that you will need to replace maybe every couple of years. Good luck.
Right. If you read what I typed above, it isn't technically ionized. The water molecules' intermolecular forces are supposedly broken.
Another way is to add salt, although for dietary purposes not desirous. The ions from the salts prevent the intermolecular forces from forming in the first place.
Water actually can be ionized though, no laws of chemistry and physics are broken. There is a small equilibrium, called the Kw, whereby water is in small equlibirium between H3O+ and OH- ions. Although small, this equilibrium does exist. The pull from the oxygen atom on the electrons shared between that and a hydrogen can be overwhelming at times and the hydrogen can actually leave and join another water molecule, leaving H3O+ and OH-, but it rapidly goes back.