From "Giving Telecoms a Competitive Edge with Web 2.0 Services":

"Casero uses BIG-IP LTM in all of our Web 2.0 solutions largely because of F5’s iRules scripting language, which gives us a great deal of flexibility to customize our customers’ application traffic management.

Once we made the decision to use iRules, it took less than an hour to do the actual design and scripting.

...

What we find is that it is essential for us to have the rich scripting at layer 7 we get from iRules. Although other solutions offer some of that capability, when we’ve tried to take advantage of it, we’ve taken a huge hit in terms of performance. And sacrificing performance by adding rules is the last thing in the world we want to do.

...

There’s a fundamental shift in the computing world with virtualization. Gone are the days when we’d have one big system with many applications running on it, and so we need a way to redirect traffic to a specific target area in this virtualized environment. That’s where the flexibility of writing iRules comes into play."

Couldn't have said it better myself.  All that, and more in the online article.