The Lolonis from last night never did quite develop the way I'd hoped. I can't recommend it except as an expriment.
I do intend to get into decanting details, soon. Recently (this past October) I heard a Sommelier discuss decanting, and relate a story about it. At one time this sommelier worked in a fancy restaurant in northern France; I don't recall the exact details at this moment. Regardless he was in charge of the very large (he described it as 10's of thousands of bottles) cellar. A special customer of the restaurant would come in once a week and order an extraordinary bottle of wine, which the sommelier would decant for him at the table. This customer tipped very well from what I understand. Well one evening the gentleman called ahead and mentioned he was having a lady friend accompany him to dinner this week before some other entertainment, and could the sommelier please select something special from the cellar for them?
I believe it was a 40-odd year old bottle of Chateneuf-du-Pape that became the selection although that deatil really isn't important. The sommelier opened a bottle of this wine and took a tiny taste to make sure it wasn't corked or otherwise spoiled - exquisite! The wine is carefully decanted and placed on the table in readiness for the gentleman.
About half an hour later, the sommelier is summoned to the gentleman's table. Expecting some sort of complement, he is suprised when the diner politely asks him to taste the wine that has been served them. Horror! The wine is slightly vinegary and entirely unappetizing. A different wine was rushed to the table and a great deal of apologizing commenced. What has happened?
Decanting. In addition to removing bottle sediment decanting allows the wine to interact with oxygen in the air. This is the same thing that "breathing" does for wine, but it happens more rapidly and thoroughly since the entire bottle contents are oxidized. Sometimes this is a good thing. An older bottle that seems very tannic and without much mouth-feel and fruit can transform dramatically after exposure to air. Some wines can take as much as an hour in the bottle to reach their full potential. However a few wines get WORSE when oxidized. I've had a couple of those (they seem to be rare) over the years. At first it seems a great wine, but after sitting for half an hour or more it becomes more tart, tannic or even bitter.
Which wines should be decanted? Unfortuately there seems to be no absolute rule. When presented with this question the sommelier in the above story gave a very Gallic shrug. It depends. In general an older wine (one specifically intended to be aged) will benefit from decanting. The varietal and process matter a great deal - unfiltered and unrefined wines benefit more than filtered/refined. Wines with more tannins seem to benefit as well. In particular Bordeaux, Barolo, Cabernet, Rioja (perhaps). Some Pinots might, and I suspect Malbec also. I've not had enough experience with Burgundy or Côte de Rhône to say, but I'd gladly try it on the latter.
To sum up - decanting doesn't always help. It does make you look elegant and refined when entertaining, and if it turns out the taste of the wine deteriorates you can tell your guests that even the real pros screw up sometimes!
Santé!