Is it reasonable to call Bitcoin a currency when its value is so volatile, and how can anyone rely on its price?

Think of it as the early days of Gold mining or the early time of the Big Bang. Things were very different and had violent swings before they settled down. Bitcoin in many people's opinion is no different. Most sane investors are looking at the 90 or 12 or even 180 day moving average graphs to get some indicative sense to where it is going in the long run (North of course).

Right now only a few 100,000s people are at it (trading/dealing in Bitcoins), and perhaps less than 50,000 active traders. So each trade/trader has somewhat a noticeable influence on the system (more so when large number of trades are involved). When a few million will be trading and there would be 100s of exchanges, then price influencing would be reduced further and the price will be less resilient to wild fluctuations.