i donated plasma for years and never had a problem. worst thing that happened was a phlebotomist fucking up a stick that resulted in a minor hematoma, but it didn't hurt or anything. if you are well hydrated before your visit and make some effort to keep your iron and protein levels up, you'll be fine.
all the parts of the machine that touch your blood are sterile plastic 1-time-use parts that are opened and disposed of in front of your eyes. the machine works in cycles, drawing some whole blood for a few minutes, separating the RBCs from the plasma, then returning your RBCs with an anticoagulant (which gives you an awesome taste in your mouth for some reason btw). as others have said, your body regenerates plasma quickly. you don't lose any RBCs or WBCs in the process, which are the more important parts of your blood.
also, it's a good reason to lay around for an hour reading a book while getting paid.