Really, this game is a huge misunderstanding. Read on to find out why.
Why is this important? Because when creating levels (at least for Doom), while the content of the levels is entirely up to the creator, what they can use is limited by what id used. Remember that both sets of levels were .wad files, and as such were limited by what Doom II did. Adding new enemies and weapons would have been impossible. Remember that new mods weren't made until Id released the source code a few years back, when enemies and weapons could now have programmed animations, attacks, and AI. And while Final Doom feels very familiar as a result, it makes up for this by being the longest, most difficult, and most intense Doom game to date.
While the storyline still isn't anything worth mentioning (basically, the same plot as Doom and Doom II, only now there are several more thingamajigs that the story is based around), it's hard to care. Hell, having been thwarted by Doomguy not just once, but TWICE, is pretty angry. In fact, there's not many opportunities where you will feel all that scared. Instead, there's lots of fighting in Final Doom. Even on the easiest difficulty setting it's no picnic, and on the tougher difficulty levels it's extremely difficult. Even so, I find some of the remarks about the difficulty a bit farfetch'd. Actually, I find the jump from Doom II to Final Doom in difficulty the same as the jump from Doom to Doom II. Sure Doom II was a difficult game, but the jump wasn't just because of more enemies, you had to deal with new enemies as well. At least with Final Doom you don't have much trouble adjusting to the enemies.
And I personally feel that Final Doom has the best level design in the series. While Mt. Erebus on the original Doom was my favorite for a long time, I have to give the nod to Mount Pain (TNT, Level 27) as my new favorite. You start out in a large room, in front of a platform with loads of Shotgun shells and a Super Shotgun in the center. Hop up and grab it, but be quick, as the ceililng above the platform comes crashing down. Several shotgun guys are in the corners of the room and will probably be taking potshots while you grab ammo. Take care of them and then explore for a bit. Two ends of the perimeter show new pathways, one of which evolves into more pathways. While some are brief staircases (one has a health pack and an imp on it), one is a larger staircase, whose platforms start rising when you step on it. It just feels amazingly cool to rush up the stairs and in perfect timing, hit the next floor just as the platforms stop rising. Up top, blast some of the chaingunners away, but watch out for the fake wall since another shotgunner will fire at you from there. Continue around the corners until you see a medipack in the center of the room. Grab it and the floor starts to lower. You'll notice a horde of imps following you from around the corner and onto the platform. With your [likely] loads of ammo, you should be able to cut some down before you run into trouble, however it's pretty likely that the majority of them will make it to the ground floor as you do. As you hit the ground, you'll take notice of two cages, both containing Arachnotrons. Here they walk around, but you can't hurt them nor can they hurt you. It isn't until one of your stray bullets accidentally strikes one of the cages, releasing them both. You can fire away with your shotgun, but while the imps fall continuously to the assault, the arachnotrons shrug off the shells while countering with their own plasma rifles. To avoid taking damage from the continuous fire, you flee around one of two corners. One has a medikit, which is nice, and the other houses a chainsaw. However, it also holds a Revenant, a walking skeleton that [comically] punches you at close range but fires either more-damaging rockets that only move along a straight trajectory, or less-damaging rockets that follow your movement. Once they're all dead, go across from the risen staircase to find another staircase, however this leads below the surface. Here the lighting is very dark and you'll be continually assaulted by imps as the acid eats your health. You can stay in the acid and lose health, which is the price of more places to move around in. Or you can stay on the sides (which are safe), but at the price of places to move around. You'll find several radiation shielding suits to help make the journey easier, however they don't last forever so you can't just joke around. A few enemies and a raising platform later, you run into the latter half of the area. Careful searching will net you a nice Plasma Rifle and even a Rocket Launcher, which are great rewards for exploring. You find more imps and are basically moving along an easy-to-solve maze. Once you find the secret wall letting you out, you find yourself in another large room with a large but thin structure in the middle. Grab the armor powerup in the corner as a platform lowers revealing some shotgunners and chaingunners. Take them down and swipe some of the rewards for yourself, not just ammo from the enemies, but more ammo and a Supercharge. Getting this, however, lowers yet another platform revealing not just shotgunners and chaingunners, but more imps as well. Take them down to see that the platform they were in housed a staircase, which appears to be the way out. Several more chaingunners are housed here, but although the lighting is dark their constant firing gives away their position. Take them down and notice that the structure in the middle dropped to reveal an Invincibility Powerup. However, you'll need it, as another horde of enemies ambushes you. While Revenants fire rockets from afar, demons move in close for the kill. Take them down and when you fire away an opening, take advantage and swiftly move while grabbing the powerup, which you'll welcome with open arms. Kill them all to keep them from impeding your progress any more and grab the red keycard in the alcove the horde rested in. Heading back up the staircase, you realize that there were two hallways outside of the main room, but you only explored one. Your hopes high that the use for this keycard lays on the other side, you step forward only to notice a sea of fireballs heading in your direction. A horde of imps came from a lowered structure while you were taking down enemies, and now stand in your way. With the red keycard in tow, however, you can now conquer the latter half of the level. And though I never got to the best part, seeing the mountain (Mt. Pain) at the end, and all the Lost Souls that appear to be infinitely spawning from it. Although you can't explore it without cheats (moving on the platform you're teleported to ends the level), you can use the IDCLIP cheat to move off the platform without ending the level and can explore the mountain. Actually, there's an enormous part to the end that you can explore. It is far and away the most open-ended space not only in Final Doom, but in the entire series. A huge lava lake, a large mountain, and lots of land to run around on. It's really good stuff. And it encompasses everything the Doom series is known for: lots of action (imp hordes, revenant and demon hordes, arachnotron ambush), creepy environments (underground acidic sewer), and memorable moments (mountain and lava lake at the end).
Keep in mind, that's just one level. While it's definitely the best one (at least IMO), there are also many more memorable level designs you'll come to remember in time. Remember the cool puzzle in the level Last Call (TNT, Level 30)? There's a series of torches in the beginning where you start, and you have to memorize the order. Because when you leave the door at the end of the corridor, you'll be presented with a large field of platforms, each with a different colored torch. If you step on the wrong platform at the wrong time, you die instantly. That's just another example. And when it isn't the levels that defines the memories, it's the enemy traps. With all this considered, I have to wonder, even if you don't think Final Doom is the best in the series, is it really that much worse? Sure it seems outdated, but so do many other classic games. Sure it seems overpriced, but if you buy this game, you have to consider the price NOW, which is undoubtedly cheap.
My advice: don't expect too much out of Final Doom; it had two fantastic predecessors to live up to, and coming off of those two Final Doom could be seen as a disappointment. Don't expect anything ultimately groundbreaking. That's all I ask. In exchange, Final Doom will give you some of the best FPS moments you'll probably ever experience.