Coinbase flags some accounts as "high risk" a week AFTER you commit funds to purchasing bitcoin at which point they refund your cash and you get no bitcoin. This understandably pisses people off, causing them to call Coinbase a scam, which I find highly unlikely, seeing that they actually have venture capital funding and that there a lot of users who have absolutely no problems with them.
The sure fire way to avoid getting flagged is to get fully level 2 verified which involves needing a credit card and a cell phone for 2 factor authentication among other things. This will enable you to buy BTC instantly.
If you are flagged, I've heard that you can request to get "whitelisted" so that it doesn't happen again.
