Grab the main tubes and give them a bit of a twist to see that they are still bonded to the aluminum lugs. Sometimes the aluminum lugs on these bikes will corrode and the tubes will detach, not good. Otherwise they are good riding bikes though pretty limber. Only the three main tubes are carbon fiber. I had a 2300 of the same type and it served me well, even in a crash. The 2120 is the triple version of the 2100.
I just sold one. Mine had straight 105 components, and was a very nice bike indeed. Unless the thing has been abused, I wouldn't worry about the carbon-aluminum joints. I understand some of the very early Trek frames had problems, but they squared that away and made lots of those frames with no problems.