My wife and I did a 3 day bike tour in November, 2012 on our (non-biking) New Zealand tour using a tandem bike. It was the first time either of us had spend any time riding a tandem. I've written an article about my experience touring on a tandem.

Some comments from the article:

A tandem is much harder to steer.

Balancing on a tandem is much trickier.

Getting started required teamwork.

Riding a bike and being able to converse and see the same things is great.