Certain fortified wines like Tawny Port, Madeira and sweeter styles of Sherry will last considerably longer than wine once opened. However, they will definately change character, lose acidity and much complexity after a relatively short amout of time. I've tested this on all three by taking tasting notes when opened, and then again in about three weeks after being opened.
They were still quite passable, but they had definately changed character and lost a noticable amount of the up front "snap" (acidity). I paired the port up with a good Montecristo No. 2 and enjoyed it very much. Over time, they will lose their edge, complexity and get pretty flabby if left for extended periods of time.
Whiskey, brandy, vodka, etc. lose a bit of ABV whenever you open them as a matter of evaporation. I have a bottle of Martell cognac and Seagrams VO that my dad gave me a few months ago that are about 2 years old. They have both lost their edge in that their signature characters and flavors have really fallen off, and the alcohol bite has dropped substantially. They're what I would call weak/watered down if I were to get one in a bar somewhere, and I'd send it back and ask for a different drink.
Hope that helps,
Oskaar