To cut a long story (see below) short - it simply depends on the quality of the sherry wine: If the cask contained high-quality (fully matured) sherry wine it is able to support a complex, harmonious and smooth malt whisky maturation while casks which contained low-quality young (unmatured) sherry wine lead to a rather spiky, battling and more simple sherry maturation style. Do not get me wrong, there are great sherried malts of both styles out there (as well as there are sub-standard sherry maturations of both styles). But the marketing guys (and gals) do not like to talk about low-quality sherry used to season modern whisky casks, understandably...
If you are interested, here is the long story (resp. history) of the different whisky maturation styles using sherry casks:
Forget about tasting a dram which matured in a cask used to mature sherry wine before, this simply will never happen. Sherry wine matures in a so-called Solera system of casks for many years until it reaches its final quality and can be sold to demanding wine connoisseurs. Depending on varying techniques used (e.g., with or without oxidation) the different sherry styles like Oloroso, Fino, PX etc. develop during this long maturation process. Because of these sophisticated techniques a Solera system is very sensitive to any change, especially to new casks as tannins can kill the quality within a Solera very quickly. For that reason no bodega ever replaces any cask in its Solera system unless it is completely broken and cannot be fixed again (in which case it is of no use to whisky distilleries either). In addition to that a Solera system cask owns almost no tannins anymore and hence does not support the whisky maturation very well (tannins are needed here). So where do the sherry casks for whisky maturation come from if they were not used to mature sherry before?
Until the mid-seventies almost all sherry wine was shipped to the destination markets (e.g., UK) using so-called transportation casks. These casks either were newly coopered or used to ferment the wine or store young (unmatured) wine before being cleaned to serve as a transportation cask. The finally matured sherry was drawn from the Solera, filled into the transportation casks, shipped and bottled upon request in the destination markets (e.g., in London, Glasgow or Edinburgh). Depending on how long the shipping and marketing of the sherry took these casks contained high-quality sherry for any period between a few weeks and several month, certainly not longer. Because it was too costly to send these casks back to Spain they were sold to the whisky distilleries once emptied which had a constant supply of excellent casks this way.
From the mid-seventies on steel bulk containers gradually replaced the transportation casks and the whisky distilleries had to use even leached sherry (refill) casks more often. To enhance these exhausted casks they remembered an old Spanish technique of producing powerful sherry flavours called Paxarette. Paxarette-improved casks lead to more simple sherry maturations than fresh transportation casks but they offer very tasty lush sherry flavours and they are able to support very long maturation times as the tannins are rather low with these exhausted casks. Unfortunately, the use of Paxarette was banned by the Scotch Whisky Association in 1989 (not by law, but by agreement).
When Spain joined the EEC (now EU) in 1982 the supply of new transportation sherry casks to whisky distilleries came to a complete stop because of a stupid EU regulation which prohibits "the bulk export of fortified wine" (which is sherry). All sherry had to be bottled in Spain instead and the whisky distilleries had to look out for a new supply of sherry casks. As Solera casks were both not available and not suited to whisky maturation they focussed on the fermentation and young-wine-storage casks instead and as whisky demand grew they contracted Spanish coopers to purposely season new casks with young (unmatured) sherry wine for any period between a few weeks up to two years (depending on what the distillery is willing to pay). This wine afterwards is sold to either (cheap) brandy distillers, producers of vinegar or it is simply flushed down the drain (as the world does not need that much vinegar and cheap brandy anyway). Because of the sherry did not mature these casks cannot be differentiated according to the different sherry maturation styles (like Oloroso, Fino, PX, Manzanilla) and hence such whiskies are called simply "sherry cask maturations".
These empty "seasoned" casks are then shipped to Scotland but because sherry-soaked wood quickly rots in hot climates like southern Spain the Spaniards used an old Roman technique to prevent this - namely burning sulphur candles inside the casks to kill the bacteria which cause the rotting. That is where all the sulphur-spoiled drams come from (you never find such sulphur flavours of spent matches, burned fireworks etc. in old-style maturations).
So, it is possible to produce old-style sherry maturations today and first-class distilleries (e.g., Glenfarclas) still source (some) casks which contained high-quality matured sherry wine before. But as bodegas try to adapt their production capacities to demand (to avoid costly excess volumes) only a few (I guess hundreds) of such casks are available and these casks are very expensive - that is what the whisky marketing departments tell you. But they do not tell you that the vast majority of all sherry casks (I guess more than 90 per cent) are seasoned with young, unmatured (low-quality) sherry wine and hence these casks are much cheaper.
Not that drams matured in such cheaper casks are by definition worse than old-style sherried drams, not at all! It simply depends on your personal preferences: If you like untamed, spiky and "raging" sherry maturations (I found that often younger drinkers prefer this style) than the modern maturations are appropriate to you. But if you prefer complex, balanced and smooth drams than the old-style sherry maturations should be your first choice. By the way, I like both (if the quality is right).
[August, 2007] Together with friends I emptied a bottle that I bought for 63 Euro in August, 2007.
In my old rating system (ten scales that translate into WB points: 50 - 66 - 75 - 80 - 82,5 - 85 - 87,5 - 90 - 92,5 - 95) I scored this 90. My notes state "Yes, this is the old Macallan style we all adore!". This bottle was gone during one night (but I saved same samples)...