I dont know about flashes but it does help alot, depends on what your doing. Seen some portrait pics wit flash used outside in daylight, helps cuts the sharp hard shadows is really all I know.
Thinkin about it too but price is hella high.
I recommend the canon brand ones, i got a nikon one for my nikon and don't regret spending $180 for it, good flash and especially love how i can use my camera's on camera flash to trigger it wirelessly... if ur ona budget tho.. the Yongnuo YN-560 is one worth checkin out.. heard very good things about it.. u can copp it here: http://shop.ebay.com/hkyongnuophotoequipment/m.html?_nkw=&_armrs=1&_from=&_ipg=&_trksid=p4340
strobist.com is the best website to learn about flashes/lighting, thats where i been learning.. i wanna cop an alienbees ringflash next or some studio strobes that i can power portable too