Platform sharing is the norm at most automakers...been that way for decades. The Cimarron was an example of bad badge engineering--pretty much a mediocre Cavalier with some slightly different trim and a longer option list.
As far as Cadillac today, the ATS and CTS are on Alpha, which is exclusive to Cadillac for now, though the 6th gen Camaro is supposed to be on it. The XTS is on Epsilon II and has a lot of dirty bits in common w/ the Lacrosse and Impala, but has it's own unique interior and exterior. The SRX is on the same platform as the Equinox and Terrain, but again, unique body and interior. The Escalade is on the same platform as GM's other fullsize BOF SUVs (Tahoe, Yukon, Suburban) but has a lot of differences inside and out...