This thread is old, but what the heck—I'll jump in. Tandems are definitely not instant gratification. When we got ours, we had no tandem experience except our training rides, where the trainer (the owner of the shop we bought from) was experienced. Yes, an inexperienced stoker can even run you off the road if you as a captain are not experienced, even though her bars don't turn. Initially, it was all I could do to keep the rubber side down. We never did crash; but after several rides, I was ready to put it in the dumpster and write it off to experience.
We did gradually get it together though (it's like you have to learn to ride bike all over again), and after a while, we could ride no-hands, track-stand at traffic lights, and she could get out of the saddle and stand on the pedals without warning me, even when I was on the aerobars. No problem. We rode it 10,000 happy miles, and then for reasons she never told me, she suddenly decided she didn't want to do that anymore. Riding with our sons was a blast—so fast!—until they got too tall to fit on the back (and I, too, am too tall to fit back there).
Regarding the "greater mass" comment made earlier: Note that tandems can stop faster than single bikes, because there's someone holding the back end down, so you can brake harder without rolling the whole bike over the front wheel like you would on a single bike. With our inexpensive Tektro mini-V rim brakes and STI levers, I can lock up even the front on pavement, wet or dry, with a single finger. It really took me by surprise when I found this out, quite unintentionally.