Well, since you described the first ones as "tedious", let me put it this way: Yes, the game can get hard. But that doesn't make it any less of a great, exciting, and very enjoyable sequel.
The storyline is wonderful and deep, it's much longer than that of the previous MD game, and I personally think it's better. Gameplay is generally unchanged, but with 4th-Gen elements added and all the new Pokemon save for Shaymin, Arceus, and Giratina's Origin Form. There is more to do, new dungeons... It's pretty much the same game, but improved a bit in gameplay, stuffed with much more storyline, and over all, longer-lasting than PMD1.
I would reccomend it. No doubt about it. Time or Darkness (it doesn't really matter which; the main differences are Lucario and Celebi or Lopunny and Mewtwo), or both, you should get this game if you enjoyed the first.