We have been using ShipWorks for about 6-7 years. It is easy to use with our Magento store and is very quick. 99%+ up-time. Occasionally there are hiccups that generally are caused by SQL server, and I experienced an episode two days ago after about two years of no issues.
I called and figured out an immediate work around with the support person. Later they called me back and remotely fixed the issue. So I was able to ship each day.
You feel powerless when you can't ship and you can't fix it yourself, and shipworks has always been great about solving the problem quickly.