Most of the ones we bought were pretty good, the only exception being cream soda, which kind of had a burnt plastic smell/taste to it, but other than that, everything else we've tried so far has been good.
Once you buy the initial setup, a CO2 cartridge goes for $30 new or $15 for a refill (and you can ship via UPS if you can't buy them locally, but usually you can.) Those are supposed to give you 120 liters worth, but even at only 100 liters, that's 15 cents per bottle, plus 40-50 cents for syrup, making your soda about 55-60 cents per liter. Pretty competitive.