It was a bit tough at the outset, although much tougher on the locals, as he made a living as a robber baron in Calabria. But by 1057 he had become Duke of Apulia and two years later Duke of Calabria. In 1081 he launched a hugely ambitious invasion of Byzantium via modern day Albania. He defeated the Emperor Alexios at Durrazzo (Dures) and conquered most of Greece, although he didn't have the resources to hold it.
This is also one of my favourite wargame armies and GDWS did a display game of the Battle of Durrazzo in 2005. I have also travelled over large parts of his route in the Balkans.
As a result I was delighted to pick up a copy in Wigtown of Finch Allibone's travelogue of her own journey in pursuit of the Guiscard. She started from his birthplace in Coutances, through Italy and on to the Balkans. She tells Guiscard's story together with her own visits to the sites of interest on the way. An entertaining read if you can find a copy.