We have a similar story. Had friends with a tandem, and thought we might like it too. While on vacation we rented a tandem (a really crappy cruiser) and rode it for several hours. We decided we would enjoy it, especially on a better bike. Started looking on Craigslist and found 3 decent tandems within a couple of weeks. Ended up buying a used Santana for $600, and luckily the frame fit us. Rode it for about a year with DT friction shifters and a 6 speed cassette, and a rear wheel which needed truing about once a month. Have since upgraded to a 9 speed with new deraileurs and indexed bar end shifters. Also had new wheels built and replaced the captains stem, both seat posts and saddles. All told, I've spent $600 initially and another $1000 in upgrades. I'd definitely recommend a good used bike that you can either resell if you buy something better, or keep and upgrade.