It's kind of funny that this came up on it's own. We were looking at becoming a dealer for Kinetic a couple of months back, and did a bit of market research. Time may tell a different story, but the general consensus was that these would never be quite as popular in the US as they are in other countries due to the availability of "live" radar data by a couple dozen different sites, like FlightAware, Passur, and so on.
While it may not be quite as live as you'd get with the SBS-1, it seems to be enough for the casual hobbyist.
As for Mode C, in addition to the issue of not knowing the registration number of the aircraft you're receiving as Jason mentioned, there's also no (easy) way for you to tell where it is. Mode-S has GPS data embedded in it - Mode C doesn't, so you need a big, spinning, PSR antenna on top of your house to determine position.