In my opinion, Doom 2 is even better than the first. Yes, theres only 1 new weapon, but the game makes up for this by including a ton of new enemies. These new enemies make the gameplay different and keep the game from getting stale. And just when I started to get a little tired of Doom 2, I got to the 3rd last level and the game got me completely back in my headspace.
The game is also significantly harder than the first Doom. Playing on Ultra-Violence in Doom wasn't to hard of a challenge for someone who's played a few old school FPS. But Doom 2 is hardcore. The way they introduce you to a new enemy, is by throwing like 8 of them at you all at once. And then after that, you get 10 or so more of a new enemy! The 2nd half is a little bit more bearable, but the 1st half of the game actually had me forcing myself to run past enemies, because I had no ammo! Beating this game feels like an amazing accomplishment and I'm glad to have done it.
If theres anything to complain about, I guess it would be that the levels aren't divided into episodes. This is perfectly fine with me, but in the first Doom, you could start from any episode and just play to the end of that set of levels, where Doom 2 doesn't have that. It doesn't give you the drop in and out option like the first does. But that really isn't a complaint at all, since console commands allow you to go to whatever level you want.
If you haven't played Doom yet, I would definitely recommend playing it first, as it gets you ready for Doom 2's difficulty. Doom 2 is one of those classics that no matter how good a sequel is, people will still prefer the original just because of how fondly they look back on it.
If you want a definitive collection of the old Doom games, get the "Doom Classic Complete" pack, as it also give you the Master Levels for Doom 2, which don't add anything new, but are altogether well designed and great fun to play.
Long Story Short: BUY THIS GAME!