Pingdom introduced "transaction monitoring" a few months back (migrated from the beta section) i.e. you can setup a complex trial of steps and validation requirements for any given number of pages on a site.
The advantages are that you can monitor all the way through to checkout whether a Magento store is working or not. This is something we deploy "as standard" to any medium-to-large client as it highlights any connectivity & design issues before its too late.
An example flow: