[poboy]

This is opponent dependant, which is why you really need to pay attention to the hands you're not in. Typically an underbet is a draw or a monster, tight players seem to underbet their draws while LAGS underbet their monsters(the complete opposite of what they should be doing). Then their are some players who always bet the same amount regardless of pot size as if they were playing limit, my favorite type of player. It's all about knowing your opponent though. JMO