It depends what you are deregulating, how you are deregulating and the structure of the market. Deregulation in the British power market was successful for the consumer but lousy for investors because there was an overabundance of generational capacity. It failed in California because it wasn't really deregulation. That's not saying true deregulation would have worked in California, but companies, for example, could not enter long-term power contracts in some markets. In Texas, deregulation has been good for consumers. As for water, people like to point to a few examples of deregulation not working, but the main reason why private companies are invited into countries is because the countries do not have the capital to upgrade their own facilities. So the option is higher water prices, but at least there's water, as opposed to no water at all.
People like to point to power and water as examples of deregulation, but people forget the 1970s and 1980s when there was tremendous deregulation occcuring in a number of industries which has been enormously beneficial for the economy as a whole. Airlines is an example. Another is natural gas, which was regulated in, I think, 1991. Gas used to be burned off at the tip because it didn't make sense to use it as an energy source. Now it is becoming more ubiquitous.
Generally, de-regulation is a good thing because it sends market signals to those areas in need of capital investment. However, when it occurs, it must be done so intelligently.