How effective are coil filters, anyway?

Just wondering if anyone has experiences/uses one of those coil filters that attach to the spout of your teapot instead of a filter-type infuser. Do they clog easily? Block the stream of tea? Allow bits of leaf into your cup? Or are they really effective?

Like you see here: http://www.rishi-tea.com/store/teaware_detail/DetailBlackTeaSetTeapot.html