I had a few transactions in a row deemed "high risk" and canceled after using coinbase successfully for awhile.  After that, I stopped using it and went back to using Dwolla->mtgox.

I think it's important to keep in mind what Coinbase is: it's a startup consisting of two guys, some furniture and a few computers.  Coinbase is quirky and has some issues now, but it will probably get better with time as the company matures.  They have bitten off a big problem and they probably just don't have the staff and experience to deal with all the issues yet.  Eventually they will stumble their way making it work better and there won't be all this BS with "high risk" canceled transaction.

Bitcoin really needs Coinbase and companies like it that make the conversion from fiat -> bitcoin easier.  Dwolla->mtgox isn't the answer for most people.

In the meantime we'll just have to tolerate the quirkiness of coinbase.