In our case we use it to test thoughtput up to 10Gbps (aka, does vendor marketing actually apply in the real world with our IOS release, ACLs, hardware, etc...) with all possible packet sizes. BGP/OSPF load testing (inject remove routes while breaking other stuff in the network), we also have an SDK which allows us to build virtual machines per Ethernet port (each one has it's own CPU) and test other components (SMTP load on mail gateways for ex). For us the sky is the limit, I do quite some work on multicast data flows and testing non RFC compliant packets to see the impact on our infrastructure.

I know that Ixia has software for VPN/Http/SSL stress testing, but my work is L2/L3 mostly so I focus on the dat paths and flows. Personally I like the gear, have access to quite a few scripts from 3rd parties that are used for regressive testing of software releases so for us it's a great way of validating an IOS release prior to deploying it in the network by checking that features we use are not broken in that release (= counters, subinterfaces, memory leaks, ....)

Thomas