Martha,
WebsterBikeMan is pretty much right on and I'll preface everything else I have to offer with the disclosure that we're the furthest thing from tandem experts having all of 400 miles. We in our early 60's and reasonably athletic.
My wife and I returned to bicycling last August after a 20+ year hiatus. We came to the conclusion that riding together on a tandem would be more enjoyable than doing 100K and 100mi rides on our singles. The search began for first a used tandem followed by test rides on new tandems.
Good used tandems do take time to find, but represent excellent value. We did find a pristine 1994 Santana Sovereign for $1500 (asking). In the end I thought we might spend as much as $1000 for seats, tires, tubes, cables, CF fork (maybe), convert barcons to STI or similar, complete tune-up, but the bike was in excellent condition, just hadn't been ridden for a long time. We were a bit spoiled by having logged miles on our Shimano 105-equipped singles which somewhat raised the bar for equipment.
We passed on that particular bike and shopped at a tandem-only bike store that handled Co-motion, Santana and daVinci. We test rode all three over the course of two visits, the Co-motion once and the Santana and daVinci twice each.
Co-motion - in test ride configuration this was the only tandem without a CF fork. Vibration through the brifters was noticeable almost immediately and became objectionable. Seemed to handle very well and judging by the number of owners would have made an excellent choice.
So it was down to the Santana and daVinci. These are both great rides in my opinion. If you locked out the ICS on a daVinci and ignored the shifting/gearing differences there wouldn't be much to choose between them. Both seem to handle very well. There is a slight weight difference (I believe), but my wife and I are not racers although we are interested in doing faster (for us) paced longer rides. DaVinci does have some proprietary parts: FD, RD, Chainrings and the ICS intermediate shaft assembly. The FD should never wear out. It is only customized ot support the four chainrings. The RD is a modified SRAM unit for compatibility with the Campy brifters. The chainrings are steel and should be long-lived. Really, its only a question of the ICS mechanism and I was willing to accept this one area of unusually non-standardization.
In my mind the two areas that define the difference are the ICS and the gearing, but I should mention that you can buy a daVinci with Ultegra STI Brifters and a triple chainring.
Gearing: I do think the quad chainring which provides very wide gearing and fast shifting is a plus. At first it doesn't seem as though you would need the wide range, (24-60 front, 11-32 rear), but a tandem unless in the hands of a pair of strong riders does need/want low gearing and on a slight downhill the 60 tooth (actually 30x2) chainring really comes in handy; these things can really fly downhill. We've since changed out the 11-32 for a 12-27 so we can keep our cadence more easily in our overlap region, but this is an individual thing. If we were riding something with anything but a short stink over 8-9% grade I'd put the 32 on, but this is an age thing!
ICS: We do pedal in phase. I can feel when my wife is slightly out of phase, but she gets in phase in a hearbeat. Usually, the out of phase situation is the result of my coasting for a second let's say for a pothole, something I shouldn't have to do. Does it hurt the handling? Not that I've noticed, but if I had one of my 200 lb. sons on the back out of phase in exactly the wrong place I'm sure I'd notice. This is almost not relevant since getting in phase is very natural. Regarding us scraping a pedal I don't know how many tandems are pedaling hard going around this sharp a turn and it's the most natural thing in the world for stoker and captain to have the pedals level or inside pedal "up" when cornering at the coast. You won't find ICS a cure-all for differences in strength. If one rider has a very circular, even pedal stroke and the other more up/down application of force it will take some time to smooth things out; I can feel when my wife is tiring on a hill in too low a cadence and if on the other extreme I get the cadence too high she can't keep up. I would think either drivetrain requires the captain, usually the stronger rider, to find a cadence at which the stoker can apply his/her balance share of the effort. When I select too low a gear on a short hill and the cadence start to drop I know I've put my wife in a situation where she can't input her "share" at that cadence - my bad.
Paceline: I agree with Merlinextraligh that pacelines are a bit difference on a daVinci. My experience is limited, but the stoker can't see the gap and I can provide direction through the drive train. Still, it's workable and not a big deal for us since mostly we are riding with singles and the dynamics are so different that I find this to be more of a challenge that the difference in the drive train.
Verbal communication - this isn't going to be much different between conventional and ICS, but I can't control the load on the derailleurs through the timing chain so I'll call out "shifting" for the RD if there is a load, but often it's enough if I unload the pedal slightly. We use "back off" for the FD where I really need the stoker to ease up, but I'm voting for something more to the point.
The DV FD is not indexed and takes a bit of getting used to, but the Ergo shifters are very popular and considered to be more robust than STI. If got to concentrate a bit when I go from my single to the tandem.
Summary: My wife and I really like the daVinci and I think the generall feel and gearing and a small price difference was probaly more of a factor than the ICS. I'm an engineer by trade, but finally stopped agonizing over the possible loss of efficiency due to the extra chain. If we werereally go-fast, yonger folks that might have weight in favor of a conventional drive train, but for use the few percent didn't make much difference. Would we have been happy with the Santana - absolutely.
Other tandems out there of course, but we bought what we could test ride from a tandem-only dealer who took very care of us.
Sorry for the long post - hope this helps.