OK this is not directly related, but I have tried a friends Canon Lightspeed (it costs more than my FZ20 :lol
You can manually set the power if you want which is nice as it is very powerful, much more than my Sunpak 544. I assume the Sigma does the same. The canon flash is very confusing to use manually with the display on the back (read: i was just pushing buttons and turning dials)
Strobe is a very nice feature,I have used this at night to get some very nice shots. Even during daylight the strobe is nice for action shots. If the strobe can work at a good power level even better. The Sunpaks strobe is a little weak at times. If you get used to using the flash manually you can just dial the power setting as with the 544. I have found that during outdoor daylight shots the exact power is not critical and works well as a filler. At night the exact power setting becomes more important. On the other featuresI have no real experience on, so cant comment.
Sorry not exactly what you were looking for, but when i saw this lonely post i had to post a reply to keep it company :-)