We did the Koridoor tour last year, as follows:

1) Third tunnel
2) Dora Observatory
3) Lunch
4) Dorasan Station
5) JSA @ Panmunjom

Regarding the tunnel, on the day we went, the parking lot at the Third Tunnel had quite a few buses in it. We walked down to the tunnel and back out, a nice little workout, but I thought the tunnel itself was anti-climactic. We were surrounded by people coming and going, so even though there really isn't that much to see down there, there was no lingering, and it's not a five-minute walk, either. I can't recall exactly how long it took to go down and climb back out, but according to the itinerary we received, they allowed about 90 minutes for that part. I probably wouldn't do it again, but if I did, for sure I would wear some reasonably waterproof hiking shoes, as there was a good amount of standing water under the rubber mats you walk on. My shoes and socks were wet at the end.

Regarding Dora Observatory, it was overcast that day, so the visibility was not good. The 3-D model of the surrounding DMZ area was probably the best part of that stop. You can get a sense of it from Google Maps, but the 3-D model is well-labeled, whereas Google Maps is not.

Not to be a complainer, but to be direct, the cafeteria-style lunch was a waste of 10,000 Won/person; I've had many such meals in Korea, and this was definitely below average. We would have enjoyed lunch much more if we'd swung by the neighborhood shijang (market) and scored some gimbap or that sort of thing before catching the tour.

By far the highlight of the trip was the JSA. The rest of the tour was pretty forgettable, but we got to see some Norks at the JSA and for whatever reason, it made it very memorable.