There have been a number of improved netwatch scripts listed on the mikrotik wiki in the past however many of these are hard to understand, broken or both.
I had a request from an associate to assist them finding a solution to fall over VPN traffic from one link to another in the event of an outage; in a network configuration where it wasn’t possible to use the local upstream router as an indication of the VPNs status, so I took the opportunity to revise a netwatch script based loosely on the one located here: http://wiki.mikrotik.com/wiki/Improved_Netwatch_II
My rewrite of this allows both the “up” and “down” scripts to be called from the same place (preferably a scheduler entry) and to be extra nice I’ve commented the whole script so you’re all welcome to modify as you see fit.