We run a WP Multisite installation which services several hundred sites (all under out control, not user sites). I was just about to set to implementing an extension to our own custom plugin to minify and combine all the JS & CSS files, when I found WP Minify that uses the same Minify engine I was looking at (Steve Clay et. al.'s).
So, I installed it as the network admin, network enabled it and ensured the cache dir was writable. Result? No effect. I played with the settings, ensured I cleared browser caches and even checked the Firefox console with FirePHP enabled in FF and Wp Minify. There is no difference between WP Minify enabled or disabled.
Hence, before I start down the path of trying to debug why WP Minify seems to be having no effect, I thought I'd ask here if anyone knows if it is compatible with WP Multisite (or has experience using it in that environment).
Ultimately, we'll be using it on a multi-server environment in which there is no shared filesystem. I plan to hook it to use memcached for the cache rather than files.