Is there any good reason to buy Bitcoins? I've read the introduction to bitcoins. But the main question remains: what reasons could push me to buy bitcoins today?
For speculation, you would buy when the price is low in anticipation that the price will rise in the future.

There are some signs that the currency is seeing wider use and has greater potential in the future.  For instance:
 - http://blockchain.info/charts/n-transactions

Concluding that that growth means that a bitcoin will be worth $4 or $2 or $32 by any certain date is more or less speculation though.  

As far as why you might want to buy bitcoins today?

How about to participate in an online lottery where the jackpot payout return is 99% of the total wagers.  Beats the heck out of doing a scratch-off at the gas station.  Tickets are about $1 each.
 - http://www.bitlotto.com

How about for the novelty.  Have you ever bought a quart of pure maple syrup direct from the farm?
 - https://bitcointalk.org/index.php?topic=55594.0
or Alpaca socks?
 - http://www.grasshillalpacas.com/alpacaproductsforbitcoinoffer.html
or a Blue Canary Night Light:
 - http://bluecanarynightlight.com

How about to prove your brilliance?  Bets of Bitcoin is a predictions market.
 - http://betsofbitco.in

Maybe you want to purchase something and would rather that purchase not show on your credit card bill.  Or not.

Or maybe you want to make a purchase from someone on Craigslist and don't know if you can trust that they'll ship your order.  There are escrow services (with dispute resolution) available using bitcoins (compare BTCrow's 1% to escrow.com's $25 minimum fee):
 - http://www.btcrow.com

Maybe you want to send a few bucks to someone in another country as an expression of appreciation.  Bitcoin transfers are nearly instant and know no borders.