SELECTING wines for a Shiraz masterclass a few weeks ago, I tried to make sense of why the variety has become such a force in the world of red wine in the past few decades.

In the late 1950s, there were fewer than 2000ha of it in France and far less than that in South Africa, California, Chile and Argentina combined. The only country with extensive plantings was Australia - and there most of the fruit was used for fortified wine.

Today, there are more than 65000ha of shiraz in France, 10000ha in South Africa and 12000ha in Argentina. The Shiraz crush in California has gone from 586 tons in the 1990s to more than 133000 tons today.

The grape that was once used to add colour to the Pinot Noirs of Burgundy and to add weight and body to the wines of the Medoc - and was otherwise largely unknown - has gone from rare to ubiquitous in little more than a generation.

Unlike Chardonnay, for example, where there is general agreement about what the variety delivers depending on the extent of its phenolic ripeness and how it has been handled in the cellar, Shiraz is more elusive. In the Northern Rhône, home to Hermitage and Côte-Rôtie, the key word is "peppery ".

In the Barossa Valley in South Australia, which has the most extensive old-vine treasure trove in the world, it often expresses itself as a juicier raspberry and plum compote. The barrels play a role: the tighter, less flamboyant French casks sometimes impart a charry caramel note to the spicier Rhône examples, whereas many of the Australian wine-makers prefer the more perfumed American oak, which adds a whiff of coconut oil to go with the butterscotch.

Growing conditions are also important, though most of the sites where Shiraz is known to flourish are warm. When the fruit has not been fully ripened, the apparent pepperiness is simply another dimension of greenness. Heavily toasted barrels can sometimes conceal this, but only for a few years.

For all its runaway success when it comes to colonising the wine world, Shiraz seems to be losing traction. The vineyards are there, and everyone is trying to make statement wines from the often rustic fruit sourced from young vines. A significant percentage of poorly sited and badly tended plants, not to mention clumsily made, porty (or weedy) representatives of the wine-maker's art, are the inevitable result of a planting boom on this scale.

In California, bulk Shiraz is becoming increasingly unsalable. South African growers are getting - in real terms - about half of what they banked in 2004.

Part of the problem, beyond the bucket-loads of inferior wine, is Shiraz's lack of identity. Some of our producers are aiming for a classical French style; others are inspired by South Australia. Sadly, their motives are not always driven by what nature has given them, but by what the marketing men say is going to sell.

Another problem is that Shiraz sounds sexier than it tastes: it may have become a fashion variety, but the austerity of the Northern Rhône is not designed to conquer the hearts and wallets of the punters. Charles Hopkins's award-winning De Grendel 2010 cleverly combines French "pepperiness" with some of the sweeter notes that come from vinifying a portion of the blend in American oak. The famous 1994 Stellenzicht would be an anachronism today, an Aussie lookalike made from almost raisined fruit, but which (to judge from the fact that it has never really been repeated) miraculously retained some freshness.

When circumstances are propitious, there's no doubt Shiraz can deliver. The best vintages of the Haskell or Eagles Nest have a floral note to go with the spice. The finest Gimblett Gravels wines from Hawkes Bay in New Zealand or the top appellations of the Northern Rhône can be breathtaking in their balance and intensity. The problem with boom-era Shiraz is the unhappy ratio between frogs and princ(ess)es.