N.L.
I used to donate platelets in college. It does take longer...about an hour and a half to two hours. I apparently have fantastic platelet counts so they did a "double" donation every time. The needle is slightly larger than the one for whole blood donations.
Depending on the collection system they have, it can be done in one or two arms. The older ones take it out of one, spin it to get the plasma/platelets, and put the rest back in through the other. The newer machines do it via one arm/one needle, I think. At the Red Cross center when I used to donate, they always let us pick a movie to watch.
Also, you may start to feel a little queasy b/c of the anticoagulant they use. I guess it affects your calcium levels or something. They used to keep calcium tums nearby for me.