That's so funny you should ask this... I've actually been working (slowly) on making a youtube video to show the difference between series/parallel/single spliting on humbuckers.
Making diagrams to show signal flow and such has been taking longer than I thought.
It might be time to get back on that.
I have 2 of my guitars wired with all it's humbuckers with the S/P/S switches. I'd say the best way to describe the difference is that parallel basically sounds like a slightly beefer single coil that's humcanceling vs series that is much more compressed. With dirt it's more like a medium/high gain switch instead of pure single coil vs humbucker sound..
In this video I switch all around. Whenever I switch the mini switches, anything in the up(towards the low strings) is series. Anything all the way down is parallel. Although a good chunk of the video is splits combined in various other positions (with the bridge always on switch)
http://www.youtube.com/watch?v=SVTWSfFNe4Q
Actually this one is much better to hear the difference for parallel/series....
In the intro I'm in parallel on the neck and you can hear the clarity and almost single coilness of the pickups before I switch to the bridge in series.
http://www.youtube.com/watch?v=LB03pPOqvJo&feature=mfu_in_order&list=UL
Can't imagine building a guitar without them anymore. I really like having the options and would actually take parallel wiring over single coil if I had to choose.