"Trace the Courier" is interesting in some sense. On a pure gameplay level, this mission is either a bore (if you save a lot) or a chore (if you do little saving). The layout is the one from "Ambush!", so nothing new there. That said, there's little focus on the actual level, since you're wasting time trailing whomever is carrying the mysterious letter around.

Despite the gameplay being bad, there's some intrigue to be found. The second courier is somewhat unsettling and creepy. He barely talks, seems to have a very high level of awareness (he can spot you easily, gets alerted by anything, etc), and walks faster than the average NPC. The sequence of him being hit by the Mechanists is interesting, and the cemetery and portal at the end are mysterious. The plot thickens, and that does increase the quality of this mission. The Expert objective is just dumb, however, and I will just tell you that I used the "end mission" key combination once I got to the portal at the end. Having to go back to pick six pockets (I had picked four or so) is just pure annoyance, and I didn't feel like I'd actually experience anything by doing that.