For us, it makes our DB server queries per second chart look like this:
The spikes are the counters being updated (1 query per thread viewed in the last hour). When you have 30,000+ unique threads being viewed every hour, it starts to be a little silly with 30,000+ SQL write queries going on.
You can make it much more efficient by JOINing against a temporary table and updating based on the JOIN and rewriting the XenForo_Model_Thread::updateThreadViews() method to use a query like this:
UPDATE xf_thread INNER JOIN ( SELECT thread_id, COUNT(*) AS total FROM xf_thread_view GROUP BY thread_id ) AS xf_tv ON (xf_tv.thread_id = xf_thread.thread_id) SET xf_thread.view_count = xf_thread.view_count + xf_tv.total