It is 100% compatible with HAProxy, and takes full advantage of the ACL functionality of HAProxy to provide fine-grained caching policy based on the content of request, response or server status.
The feature list is long. Click through to see 'em all.
Nuster is very fast, some test shows nuster is almost three times faster than nginx when both using single core, and nearly two times faster than nginx and three times faster than varnish when using all cores.
Here's a detailed benchmark backing up these claims.