Even though Doom and Doom II: Hell on Earth were great... Final Doom will leave you sick of the expierence.
The game has two campaigns, TNT and The Plutonia Experiment. Both campaigns sound interesting, but they offer no real story for a back-bone... just levels with no meaning at all. I mean sure killing hundreds of monsters is fun, it really is... but it's over and over again... and that's all you'll get from this title. When I finished both campaigns I felt no feeling of accomplishment... due to the fact that it just sends you back to the main menu, with no "Well done!" or "Congratulation!"... all you'll feel is that your time has been wasted.
One last thing, is the date at which it was released, 1996. Doom looked amazing in the early 1990's, when it was first released... but after three titles, the game stuck to its guns and it received no 'face-lift'. The graphics and the engine began to show signs of aging in Doom II, but Final Doom did'nt seem to take the hint, as it's exactly like it's predecessors... so only purchase it if you really want to.