Luigi's Mansion is so underrated. I still don't understand all the hate it gets.
The game IS pretty short, though. I was able to finish it in one sitting with all the boos in about 4 hours, similar to James.
A good way to get more out of the game is to try to get the A rank mansion at the end. Finding money ghosts, golden mice, hidden treasure rooms, watering plants, etc. are things that you could have completely missed on a regular run. Some people might think it's a chore to try to interact with practically everything in the mansion, but I saw it more as an opportunity for exploration. Just take it slow and easy and it becomes much more enjoyable.
Also, for people who like an extra challenge, you get different frames for the portrait ghosts in E. Gadd's lab depending on how well you do while catching them. For regular portrait ghosts, you have to drain all their health in a single try. For bosses, you have to beat them without losing any health (or at least finish the battle with full health). It's a lot harder than it sounds! The reward may seem kind of superfluous, but really the challenge should be its own reward.