Thebeernut and Sean seem to be suggesting that price competition is good, end of story. However, abusing a dominant position to engage in predatory pricing is generally recognised by free marketeers as being bad for competition in the long run.
Take the example of the Dublin cork air route. Aer arann used to provide a decent regular service several times a day at a reasonable price. Ryanair decided to move in on the route and was operating 6 or 7 flights each way per day at incredibly low, unprofitable fares. Ryanair could afford to engage in predatory pricing due to its size as unprofitable routes can be subsidised by profitable routes elsewhere. Eventually aer arann was forced to pull its Dublin cork route. You'd think ryanair would maintain or increase its frequency on the route to serve the aer arann passengers but No! There is now only only flight per day on the route meaning that day trips for business are no longer practical between our two largest cities. In this case, the customer loses.