What's the best Bitcoin exchange to buy with? In the early days I stayed away because a lot of the exchanges seemed to have really poor security, but it looks like it may be time to invest somewhere.
You don't need to keep bitcoins on an exchange. It's not like buying stocks or gold certificates. You can own bitcoin like having gold coins shipped to you. Even if the exchange is hacked, your usb stick isn't. Simply buy and withdraw immediately to your encrypted personal wallet.
Coinbase is great, it has been very secure and reliable. The only thing that i don't like about it is the fact that you have to wait a little while for your Bitcoins to come in, although I haven't used any other exchanges so I don't know if this is standard.
I recommend LocalBitcoins.com. Due to fraud prevention I found the big exchanges overly complicated to join. LB allows simple same day transactions that only require a quick trip to the bank to transfer cash.
My employer Payward runs Kraken.com which is a fairly new exchange that also supports other cryptocurrencies. It's easy for customers in some US states, possible for those in others, and super simple for all of Europe. More jurisdictions on the way...
using bitcoin-otc as an exchange is pretty cool too: you do it through IRC (no web interface), there is a very well done WoT, and the wiki is excellent.
If you are in the EU, bitstamp is great.
btc-e.com processes USD deposits within 24h,
great exchange to buy for the lowest price