Question: Why spend money on a level pack if you can get thousands of levels on the internet? Answer: QUALITY.
Final Doom is, as already stated, a collection of 64 new levels for Doom 2. They do not require the original Doom 2 to work (But if you're looking at this review it is assumed you have already finished Doom 2 at least once in your life), so even if you don't have a copy of the game you can play them anyway.
But the question is: why spend money on a level pack if you can get thousands of levels on the internet? The answer is simple: QUALITY. The Plutonia Experiment, the second of the two WADs, is perhaps even better than Doom itself. Because, even if Doom is indeed a very fun game, episode 1 is mainly just Open the door, shoot the zombies, open the other door, find the exit room and win. These levels have more depth in them. They have a good atmosphere and are well designed. Also they are much harder than the Doom 2 ones to the point you encounter an Archvile in the first level. TNT also has good levels but it's nowhere near Plutonia in terms of quality (however it does have brilliant new music, whereas Plutonia uses music from doom 1 and doom 2).
Even though this game does not add any new enemy or weapon, who cares? This is how we all like Doom. The new levels are enough to give a challenge to even the best doomers out there. While TNT may have some levels that are not that great, ALL levels in Plutonia are great, and the replay value is as good as the Doom 2 one, if not better.
So in the end, if you liked Doom 2 and want more, this is what you need. While you're at it I suggest you also get The Master Levels for Doom 2, which is another addon with new (mostly) good levels.
I will never understand why people think a game to be good must be "innovative". That is just plain wrong. A game in order be good has to be fun. And this is the case here. If you are really looking for an innovative game you can always buy one of these modern games that come out today that have absolutely no soul in them (there are of course exceptions but it's mostly like that).