Whenever I attend a F5 customer or partner gathering, I always ask of those who use iRules, 'Do you deploy iRules due to BIG-IP not having a particular feature or because you need to solve a specific issue within your unique architecture?' Overwhelmingly, the answer is to address something exclusive to the environment.
An iRule is a powerful and flexible feature of BIG-IP devices based on F5's exclusive TMOS architecture. iRules provide customers with unprecedented control to directly manipulate and manage any IP application traffic and enables administrators to customize how you intercept, inspect, transform, and direct inbound or outbound application traffic. iRules is an Event Drivenscripting language which means that you'll be writing code based off of specific Events that occur within the context of the connections being passed through the Virtual IP your iRule is applied to.
So while iRules might not be able to make your coffee in the morning - unless of course it is a slew of IP enabled coffee machines - they can help organizations create extremely agile, flexible and secure environments. Like Oreos and Reese's, there have been a bunch of imitators but nothing is as good as the original.