My wife and I tandem commute. It's getting sporadic now that the temps have dropped, and it's dark when we leave in the morning and by the time we get home at night, but we were doing it pretty regularly in the Summer. Averaging about 3 times per week, depending on when we needed to pick up my son at his mom's, 35 mi round trip.
We got a used 80s Burley Duet tandem in the late Spring. Started riding it around locally. By that time, I was commuting to work solo, and once we'd done some longer rides, we started commuting on the tandem. I can't tell you how much fun it is. We're lucky enough to work for the same company, so it is a perfect situation. We get there faster on the tandem than I do solo, breaking my summer goal of 1 hr for the 17.5 mi to get to work... which is only about 20 min slower than if we took the car. Other than a rash of flats that we solved with a pair of Schwalbe Marathons, and one attempted right hook incident, it's been clear sailing.
Our company has showers, but we usually skip them and just change clothes. Panniers were a birthday present and have been a godsend--the bike came with a rack. No fenders, and we're using "be seen" lights at the moment. If we get a bit of spare cash at some point, we'll go generator front hub with lighting and fenders for a bulletproof commuter.
My wife Val surprised herself and me by really getting into it. She had a cruiser she rode occasionally, but wasn't really into bikes in any big way, certainly not commuting or in any utility sense. Now that we've morphed the bike into what suits us, it's got a bit of a dual personality issue--drop bars and brooks for the captain, risers with bar ends and a wide comfort saddle for the stoker--but it works and my stoker is happy, which is most of what counts. I'm not sure that she'd have done the commute on her own, or that it would work with both of us on solo bikes, so the tandem has been key to getting her commuting.
Even buying used, it was a substantial investment for us, and I would never have bought it if she hadn't agreed to give it a try. But we were both surprised at how much fun it can be and that we actually started commuting on it. I've heard tandems referred to as "divorcycles," but we are one of those couples who seem to mesh well.
Tandem commuting is highly recommended...