I would avoid Coinbase. My experience is that BTC purchases with them take 5 days. You don't know what BTC value will be in 5 days. If the transaction is favorable to Coinbase the transaction goes through. If it's favorable to you, (a lot of BTC per $), then they say you've been flagged as high risk and cancel it at the last minute.
This is completely wrong. Coinbase is legitimate and reputable and I've used them for about a dozen purchases. In fact, one of the main reasons to use Coinbase is because you lock in the price at the time that you place your order and the bitcoins are then later released to you after the ACH transfer clears. Compare that to say, using Dwolla and Mt.Gox, where it takes just as long, if not longer, and you'll be paying the price a week from today instead of today's price.
I opened an account with Coinbase a couple weeks back. I made 2 small buys so far. The first one went well. On the second purchase I bought right before the price went way back up and they sent me a notice that they had to cancel that purchase because they thought it was high risk somehow. They suggested trying back in a few weeks.
Usually if you contact them about a cancelled transaction they'll push it through. They're still training their fraud detection algorithm so that happens sometimes.
+1
I've had this happen a few times and it seems to happen on transactions for <1BTC. Sometimes it takes longer than I would like, but they've always restored the transaction when I requested it. The real trick is to ask for your account to be white-listed, to avoid having your orders caught by the fraud detection system.
Ask to have your account white listed? Contact them when a transaction doesn't go through? what are you suppose to do, email them?