It's not real scary or spooky. It is a long tunnel that is poorly lite. It's rather "industrial" looking. There are pipes alone the walls. In places supplies for the Tomorrowland Terrace are stacked up against the wall. That's the purpose of the tunnel; to give an offstage access point to the Tomorrowland Terrace and Buzz Lightyear building.
The Rocket Rod queue is usually open on days when they are releasing new pins. They use it as a waiting area for those wanting to buy pins at the Little Green Men store.
The Tomorrowland tunnel is kind of creepy as it is very dark compared to the WDW utiladors. Very few people use it so you are usually alone in there. All left over food from Tomorrowland Terrace is down there waiting to be hauled off for compost and the trash compactor for the restaurant is there as well so it doesn't have the most pleasant smell.