I'm working with Sitecore at a Sitecore partner for three months now, and I really like it. Ok, the Desktop interface is slow as hell but the whole system is generally of pretty high quality. For my graduation paper I've build a high performance plug-in system for it and I replaced some of the parts of Sitecore to output something other then HTM and it works flawless.
The company I'm working for has build many high-traffic enterprise websites with Sitecore, and from what I know the websites are handling high volumes of traffic without any problems.