Does rum get better after opening and breathing?

It's definitely feasible as this is a 'must' for wine. Aeration has multiple benefits and affects. A produced liquid (wine or spirit) is sealed in its vessel with an inert gas in the head space to prevent contamination from ambient air, stabilization of the product to the producers (wine master, brewer, blender) desired effect. With wine, the aeration has a more immediate affect, noticeable within 20 - 40 minutes of decanting. Molly Dooker even promotes the "Shake" to drastically enhance this on their Boxer Syrah. Now, I would never say to agitate a quality spirit, but definitely think that allowing the product to be exposed and opened to air will expose flavours and notes that would not be recognized in the first pour. Spirits are much more stable than beer and wine, which will loose their quality in a short period. Say a day for open beer and perhaps 2-3 for wine (not withholding preserving applications). I've not played with opening, decanting or exposing any of my rums for the purpose of improvement. As most, I typically have a number of 'opened' bottles, re-capped and sitting in the cabinet, waiting to be shared with friends or revisited on occasion. It's not your imagination. Think about being stuck indoors for an extended period of time. When you venture outside or open a window, a deep breath of fresh air invigorates your senses. Getting that elixir out of confinement, pouring and swirling to wake it up and move the air around the molecules gets them moving. Exposing characters of what made them (oak, terrior, etc). MMMMMMmakes me want to go drink. Too bad it's 8:30 AM here. Cheers & Merry Chistmas

Yes.

I poured a double Diplomatico RE into a large wine glass which has lasted 3 hours before and after Xmas dinner including a glass of red in between. The best answer you can get is to try for yourself. This seems to be getting better the longer it's in there. My original review of the DRE has now gone from 8 to 9.

Another associates question I've had: does rum go old after opening? I could see how a day/week/month might give rum time to breath, but what about a year or more? I certainly have rums I've been 'saving' for a long time after the first taste, would be a shame if quality actually decreased over the years :(

As far as aeration goes, I definitely notice an improvement with some rums (especially the Ron Centenarios) after about 15-20 minutes in the glass. I don't know from experience if an open rum goes downhill over a period of years, but I've read they should last a very long time if corked properly and stored in a somewhat cool dark place.

I just bought some La Hechicera which I was disappointed with upon first opening it. Two days later I tried it again and was impressed.

Interesting discussion... I think that letting rum sit in a glass for a few (10-20) minutes before drinking makes it smoother (less harsh), however I have also noticed that the flavour of some of my more aged (12 yr olds) rums worsens over time - becoming more alcohol-ly and thin. Neisson Extra Old and Admiral Rodney come to mind... Perhaps they're getting too much light.

Oxygen or air will always have an effect on spirits or anything for that matter. As the bottle sadly runs low it's been exposed to more air and it has had ample time to mingle. The alcohol will become less harsh as it's vapors escape and oxidation starts to kick in. A bottle running low that has sat around for a long time can taste flat. If it's a bottle I'm familiar with that I corked than It's an on going journey and I don't mind it as much, but if I'm at a bar and paying inflated prices for a spirit I always thinks about the level the bottle is at as well as how popular the spirit is according to the bar I'm at! Macallan 12 is a perfect example. At certain bars it doesn't move much and if the bottle is less than half a flag goes up, if it's less than a quarter I'm looking at other options. Look towards wine. It's a fragile delicate beauty that has a whole obsession built into it's interactions with air. Distilled spirits or any other liquids are not immune to the effects of air.