I also own Final DOOM. This is a great piece. FD had some of the best levels ever designed on the DOOM Engine. I was truly impressed with what they pulled off when I got it. Invisible bridges, see-through doors. Bulletproof windows. Okay, those are all the same trick, but it was amazing how they used a Bug in the engine to pull off some amazing stuff.
Plus the detail in the levels was levels higher than what was in the original two.