This.
Be aware that most roles are non-tipped positions, and CMs in non-tipped roles can get in trouble for accepting tips. Honestly, a simple "thank you" can really mean a lot. I can also attest that feedback given to Guest Relations and Guest Communications does reach the Cast Member in question...after it goes by several levels of management (which is usually a good thing).
Please get the CM's name--we change locations frequently throughout the day, and "the Cast Member taking Fastpasses this afternoon" doesn't really help much. I once had a Guest try to leave positive feedback for the "safari driver that was wearing glasses." When I asked for more information to try to identify the CM, I got "he had short hair."
Incidentally, if you're planning on giving feedback, it couldn't hurt to verify that the CM is wearing his/her real nametag. Even the best CMs forget theirs once in a while and become Chris or Pat from Orlando for a day or two.