Finding the right addresses to monitor - this seems to work
As others have noted I found that whatever address I chose in my ISPs' networks, I got a lot of false negatives, which is a real pain for gaming in particular.
I tried their DNS servers, the first router in their network and some other addresses like their mail servers as well, but whatever I used, pfsense would mark the link down sometimes as much of 5 or 6 times an hour.
I have now switched to montoring my internet IP address on the routers. This disappears if the link goes down, but does not seem to generate false negatives (or at least a lot fewer - only been running for 12 hours ::) )
I am running on version 1.0.1, haven't tried the latest build with the fancy failover capabilities yet, but I'm not sure they would help me much ;)
This won't work for static IP configurations. You really should test the lates snapshot. The timeouts have been raised to make links go down not that fast.