I also like the idea of using different bikes now and then. Commuting is the same route, every day. Mixing it up with a different bike now and then helps keep it fresh.
Right now I'm really enjoying this one:
I think [MENTION=152773]noglider[/MENTION] said something about old Raleighs that I like, something about it feels like you're between the wheels rather than on top of them, I think. The day after reading that I was riding, and thinking, yeah, that's it!
Putting the load up front on a lowish trail bike makes it easy to get out of the saddle and push hard to get across busy intersections quickly, and for the two small hills that I encounter every day.
And don't forget when you're commuting...