The Splendid Table (a food and wine show on NPR) had an interesting segment featuring Randall Graham (of Bonny Doon Vineyards). He talks about Syrah versus Shiraz, and the difference in style between French (or Old World) syrah and Australian (or New World) Shiraz.
He certainly has some valid points. I did find that I got bored with the very ripe, over-extract, Australian Shiraz that I was buying a few years ago, and tend to prefer the more complex stuff now. However, I think there is a place for both Old World and New World wine, and I have had some truly memorable Shiraz from Aussie (2002 Larrikin, 2004 Losy Highway Stella's Garden, 2005 Mitolo G.A.M. for example). South Africa also makes some great Shiraz wines, somewhat in-between old and new world in style...
So, for me, it's less about the country the wine comes from, or whether it says Syrah or Shiraz on the label - it's about what is inside the bottle.
You can listen to the audio segment here and find more info on the show's webpage here.
You can also read an excerpt from Randall Graham's book.