On the same day that I opened (and eventually drainpoured) a 2011 vintage of Westvleteren XII, my disappointment drove me to immediately open another old quadrupel, a 2012 vintage of Chimay Grande Réserve.
In contrast to the XII, the Grande Réserve 2012 is a delight. It has retained much more of its characteristic carbonation than the former, while it exhibits similar oxidised flavours, all rum, old wood and leather. There's dried fruit, particularly raison, but also the warm candied citrus and spice of winter baking. There are darker malt notes too - the subtle chocolate I often find in the style is probably more prominent, while I feel the yeasty bread aromatics have actually dissipated compared to the original. The main effect is of a beer that is darker and sweeter on the nose, but with some medium sherry-like vinous age thrown onto the palate. The carbonation has dissipated some - it's not as effervescent as it should be, and this does give it a slightly thicker and more sluggish feel, but it's just lively enough to remain true to its digestible roots.
The point is elucidated best in Brew Like a Monk, where Stan Hieronymus relays a brewing principle key to the Trappist brewers and secular abbey stylists alike; digestibility. Regardless of the size and strength of the beer it should be drinkable - it should never feel heavy or thick, it should function well as a digestif after a full meal, without demanding too much of the drinker's gut. At least, this is my interpretation of the term, and it summarises my feelings about well-made examples of this type of beer. Both beers have this quality in spades when fresh, but the Wesvleteren seemed to lose every shred of digestibility in the nine intervening years, while the Chimay retains it, at least in the most part. It has developed over the years - it's richer, and arguably more complex than a fresh example.
There are some differences between the two to consider before a fair comparison can be made; the Chimay is of course a year younger and is packaged in a 750ml cork and cage bottle, rather than a 330ml with a crown cap. They are different beers - the Chimay/Westvleteren comparison is never going to be as closely run as a St Bernardus/Westvleteren comparison, but the two occupy the same ballpark to my mind. Crucial is the difference in price and scarcity - in the United States, they call this kind of thing a shelf turd. No hype, old world, readily available. I've spent enough money on expensive, rare beer from incredibly good breweries recently* that have all gone directly down the drain, to know that every beer can be a shelf turd if you treat it poorly enough.
This Chimay lived its eight year lifespan a few inches away from the Westvleteren in the same cupboard, subjected to the same conditions.
It's certainly no turd.
*the Double Barrel-aged Harmonic Convergence (the original is astounding) from Galway Bay and both Sound of Islay and Sound of Kentucky from Buxton