The first mention of bitcoin on HN was 4 and a half years ago. You could have bought them at about $0.05 back then. I wish I'd been paying more attention.
The easiest way back then was to try to figure out how to install the GPU miner and then just hope that the block difficulty did spike because everyone else was doing the same thing. When I wen't back to start selling bitcoins when it started going north of 50 bucks a coin, it was too easy, Mtgox did a direct wire to my bank account and I was limited to a certain amount per day or month (can't remember).
As for buying it now: Right now the bet is whether or not you think that bitcoin will survive governments worldwide trying to clamp down on it and whether or not you think it will scale to billions of transactions a day. I put the first one on about 10% chance of happening and the second one on about 30% chance of happening, so about 3% chance total. If it hits that 3% the maximum I think bitcoin could be worth is about ten million dollars a coin ($10k average purchasing power for a couple billion people). So I made my profit already (on my original bet that it would gain momentum, which it has) and now I'm riding a decently likely (given the payout) lottery ticket.
$10,000,000 x 21,000,000 = $210,000,000,000,000
Does the world even have that kind of wealth?
I may be a reluctant Bitcoin user sometime in the future, but attempting to profit from its price movements is an inherently risky business, especially given the legal and tax grey area it entails in many parts of the world.
If I'd held on to the BTC I bought in 2011 it'd be worth £20k right now, but I'm still glad I didn't, for all these reasons
Edit:
>>> btc_count = Decimal("100") / Decimal("0.05")
>>> int(btc_count)
2000
>>> float(btc_count * Decimal("400"))
800000.0