System Metrics displays primitive bar graphs on all pages that show a table of metrics. The bars are shown in red if they are considered slow and green otherwise. The threshold for considering a metric slow was arbitrarily chosen and hardcoded into the application. There should be a way to configure the slow threshold time for different kinds of metrics.
Since System Metrics uses Active Record to store the metrics, all of the SQL statements involved are logged among the other logging of your application. It would be nice to be able to filter these out since most developers won't care about what System Metrics is doing and will find its filling of the log files annoying.