Gally the way to think of it is comparing Bitcoin to gold like chip says.
Yes you can mine it. But you can mine gold. Gold has a current value all over the world. If you dug down in your garden and suddenly found 10000 tons of gold. It would become less valuable unless you withheld it and didn't tell anyone. If you tried to sell it, you would probably fill the demand quite easily and have to resort to selling for less. Making the whole currency worth less.
Same applies to BTC. However by it's nature there are only a finite number of them (I don't know if anyone knows how many though).
It has a value because people are speculating, and trading with it. I think there must be a massive money laundering issue with it since I'm not aware of any way to combat that? If one day we take up BTC as a main currency (let's face it, less money going through the banks merchant systems means more opportunity elsewhere however how do you regulate it??
With regards to the other coins, it's almost like they're different metals to BTC's gold (that is me propagating the notion that BTC is the best coin). Stuff like Bristol coin or whatever they came out with recently is like IMO saying "look guys, I found a new metal, it's called Tin. Who wants to help mine it?!"
Again there's only a finite number of them, but I don't think the world needs Bristol coin very much, so why would anyone pay anything for it.
I don't think there's a place for it in main society at the moment, and who's to say bitcoin banks won't emerge? Where they offer a bit more insurance on transactions in exchange for a transaction fee.