Coinbase is the gold standard in virtual currency exchangers, exclusively dealing with Bitcoin, Ethereum and recently Litecoin. They are a fast growing darling of Silicon Valley and have the heavyweight VC's like Andreesen Horowitz / DFJ backing their later stage rounds. While they're busy burning through millions of cash, they still have not figured out how to sufficiently scale and manage their customer base.
The platform works nearly flawlessly once it has been set up properly. However, I found this process to be quite cumbersome, because of the automation that goes into the process and the severely lacking human capital for customer support.
Something as simple as updating a social security number was quite painful, with repeated error messages. Contacting customer support yielded 1 response out of 4 attempts, in a 1 month period. That response simply stated something to the effect of "sorry it took us so long to respond, are you still experiencing this problem? Please try again."
The problem eventually resolved on its own, mysteriously, only to come back up again (and restrict my account) in the process. When it works the exchange occurs very quickly, and the funds can be deposited within 2 business days to an account. They can process a high volume and a large dollar amount of transactions. There's a maximum sell limit that replenishes in real time. The exchange fees are quite high, but there's a problem with an oversupply of ledger recordings and the higher fees tend to get processed faster.