It looks like the page cache is working, it just may be expiring. If I run 5 tests, it looks like the first test is slow but the next 4 all return fast: http://www.webpagetest.org/result/110531_6A_QMVN/
There are quite a few knobs you can tweak on the “Page Cache” page, including a “preload” option that updates cached pages in the background (otherwise the first visitor pays the price of building the cache for everyone else).
Enabling the database and object caches may also help improve the performance for the page visits when the page itself isn’t cached yet.
Looks like you might be able to tune the css minification settings to combine the 4 different css files for the next set of improvements but it looks like the site is doing pretty good when the page is coming from cache.