I don't claim to be correct in most of my answers and this is purely my take on things (I also have minimal formal education in economics), but I'll gladly add my 2¢

Question: so why is everyone calculating "nothing" on thoes radeons -> to have scaricity/run transactions -> ok but then you create currency with no backing at all [normally currencies are tied to something, at least say state *buy* homes/loans from banks and pays them therefore introducing new/printed money to the market
Well technically there is nothing to back them aside from their perceived worth, and their momentum as a legitimate currency as bitcoin (hopefully) gains more widespread acceptance.
Also one could argue that they are backed by the electricity used to generate each one, which is a non-negligible amount. You could even go as far as to say they are "commoditized e-tokens" which conveniently package the use of electricity for re-sale, but this is kind of a stretch and is just sort of how I look at it. I suppose the actual cost of generating each coin is indeterminate, and the earliest coins would have used significantly less energy than the ones generated after difficulty has been 400,000+
Q: how big is the economy?
I think that most of the market force right now is speculation, as it seems there are still relatively few outlets selling physical goods and accepting bitcoins as payment; you can get lots of services for bitcoins though.
Q: is it all legal?
I think this is still open to interpretation and depends where you live and what you're doing with your coins. If you really want to play dumb it's all just a "game" and you're sending e-tokens to other people that have no bearing in the real world. Obviously this isn't really true and I suppose there are likely tax and other legal considerations to take into mind when buying/selling bitcoins and stuff, but at the same time I think bitcoin still under-the-radar enough that you're not going to get an IRS audit for selling a few bitcoins here and there.
Q: who will buy bitcoins from you
From my own personal experience, I've been conducting a lot of my business on IRC (#bitcoin-otc on freenode), just directly selling coins to other people who will give me USD in my paypal account for said coins. Mtgox is still selling your coins to other people, but in a more indirect and impersonal way.
Q: who can sell you bitcoins
Same as above basically; or you can mine them yourself
Q: is it usable long-term, the price is volatile
Like I said above, I think the main market forces are almost purely speculation right now, and this is coupled with the large difficulty jumps recently sort of propping the price up due to short term scarcity. I don't think that bitcoins have enough real world use to really be worth $9 each at the moment, but it would seem that most of the commerce right now comes from just buying and selling the coins directly, and the market is what determines the price...
Q: how long will it run before the project "falls part"? Why you think it really should survice say next 5 years?
"Falls apart"? I don't think anyone really has an answer to that right now. The difficulty keeps going up which seems to indicate growing interest, and the web traffic to the guides I've written to get started with mining only keeps going up and up. A lot of this may depend on the legal ramifications too, but if governments start outright banning bitcoin that is likely to just make it even more popular. But right now...with the price so high, I would not say that bitcoins are good for a long term stable investment, but they are definitely something to keep an eye on, and make some money on the side with as well if you are able
