Deposits and withdraws are done via ash Push/Pull initiated by coinbase. Selling bitcoins usually takes 2-3 business days for the money to appear in your account. Buying bitcoins takes 3-5 days normally, though an instant buy feature is available if you have a credit card as a backup funding source.
Coinbase is a very well capitalized company. They have been around a while and do alot of business. In recent times they have been growing very quickly and as such their support has often lacked. I have used this service many times in the past though without any issues and do highly recommend their service.