...just had 'internal server error' messages on the site (including admin when I tried to log in) which was scary. Found a page that was already logged in to WPSC and switched super-caching off - now it's fine again.
The cacheing is great when it works, but it doesn't seem to work often - the cache was filled up earlier today (presumably by a spider) and it worked great, but all the files were gone an hour later (i had expiry time set to 360000 seconds)