I'm very pleased with WP Super Cache. Certainly in the last few releases, getting it to perform well out-of-the-box has become much simpler. Especially the support for CNS (self-hosted or "real" CDNs) is fantastic. However: I have a site that has some content over https (mostly the wp-admin area, including logins and registrations) and I'd like to be able to also selectively add just a few more pages to SSL as well, while keeping the rest, the majority over regular HTTP. Pages like event reservations, contact form, profile pages on the front end to either show or modify one's profile data; those seem "sensible" pages for SSL.
The thing with CDN as it currently works in WP Super Cache is that it quasi-randomly assigns the subdomains defined in the CDN settings to static elements (e.g. images, scripts, css) on a cached page. And because of the restrictions on subdomains with SSL certificates (very few have a relatively expensive "wildcard" cert which supports subdomains) that means a page "hard-set" to be served over SSL (e.g. via the great WordPress HTTPS plugin) that that page won't "validate" or more precisely: it'll have a mixed content warning problem, due to those CDN subdomains not running over a properly signed certificate.
So my suggested idea is to have one extra setting in the CDN settings page, essentially to switch off the CDN function (and "advanced" caching) for those pages that are running over SSL.
While this is an issue that few WP Super Cache sites will have (not many use a "real" private SSL cert) I think that for sites on a tight budget (ours is for a non-profit) and therefor using a self-hosted CDN as well as one SSL cert this would be a great feature.
Then again, I acknowledge that this is very much a fringe relevance issue...