I have real Cisco routers and switches but most of the time I rather use virtual devices based on Dynamips. They are portable with my laptop, don't consume additional electricity and provide me with and environment to test features anywhere I go with my laptop. I use it to test QoS, MPLS, IP routing (with OSPF, EIGRP, BGP), gatekeepers configurations, load-balancing, etc. It is just as having the real thing with you.
For example, during my last trip I found myself running 15 routers on my laptop, doing a BGP load balancing configuration validation for a service provider. (Well, I managed to do the job but my laptop's battery was totally drained in little bit more than an hour). But the idea is that, it is a really flexible solution. (BTW, I run Linux so I don't have the memory limitations that the Windows instances have.)