I know this is a pretty basic question, but my searching hasn't turned up much. It seems a fair number of people here are using CDN's to improve the speed and security of their forums. I'm curious to know how one would go about measuring whether they'd be better off implementing a CDN service or upgrading their dedicated server. Those who use CDN's, do they still use server caching? Can you serve your site over SSL via a CDN? What are the downsides to a CDN service, assuming uptime is acceptable?